Basically this question, can any Islamic states on Iberian peninsulla survives on long term until the (unrecognizable) 20th century? Or even absorbing the Christian Kingdoms?
At the exception of a possible, if not that likely, "Greater Morroco" expanding on both sides of the sea up to including part of IOTL Andalusia region, I'd say no.
Is this scenario inevitable? That once Castille, Aragon, Portugal, and such Christian kingdoms are knocked out
The goal of Arabo-Andalusian states was never to crush and destroy Christian political entities for the sake of it.
You have several occurences of the Arabo-Andalusian hegemon being established on them (for instance, the Arabo-Berber presence before 720's in Asturias; or Almanzor's campaigns and garrisons in Leon).
The main relationships with these kingdoms were essentially annual or at least regulars raids against Christians, associated with jihâd, and on which was based the legitimacy of the emirs/caliphes/Islamic rulers in general.
At first, the hegemon on North-Western principalities and regions (on the Cantabrian Highlands and Pyrenees) wasn't obviously different from what was excerced in other parts of early Al-Andalus : the pricipality of Tudmir ibn Godo; principality of Ajuan in Portugal, and of course "Arabo-Islamic" Septimania with Narbonne/Arbûna being the only real Arabo-Berber garrison north of Pyrenees.
What essentially made it were (Sorry it's a bit long, but the conquest itself didn't meant an unstoppable expansion and reinforcement, and it's to be pointed out to understand Early Al-Andalus geostrategical context) :
- A greater independence from Central and Southern Spain that existed during the Visigothic Era as well : Vascons, contained by the duchies of Cantabria and Asturias on the West, made periodical raids that the kings of Toledo never managed to fully stop.
It's the very same behavior and resistance against peninsular power that you have in early Al-Andalusian times., only strengthened by the presence of gothic refugees and a policu of fortifications.
Being too poor and hard to takeover to justify full scale campaigns, while you had more pressing issues (see below), it was let to rot. Except it didn't.
- The Great Berber Revolt of 739/740. Berbers, while representating the bulk of Arabo-Islamic forces, were treated as dirt by most of Arab nobles and trusted with either less interesting lands, or bordering lands.
When they rebelled
en masse, they simply abandoned most of northern places that Asturo-Cantabrians took back without too much trouble.
Giving the long harsh treatment of Berbers, treated fiscally (and culturally in large part) as non-Islamized peoples in spite of their conversion, the revolt is pretty much a given even before the conquest of Spain (actually, it could be easily argued that this conquest, and the transfert of wealth to African Berbers probably delayed it).
- Strategical pressure, from Franks and Abassids. As pointed out earlier, Gothia was badly defended, and Franks (roughly unified around Peppinids since the end of VIIth century, in spite of civil war in 714) were pretty much interested on putting an end to Arabo-Berbers raids on Gaul : the most famous raid is 732's, mostly because it was blown out of proportion by Carolingian hagiographs, but you had regular raids.
Some victorious (718-720 in Gothia, 725 in Burgundy and up to Sens, 731 in Aquitaine, 736 in Provence, 760 and 787 in Provence), some defeated (721 in Aquitaine, 732 in Northern Aquitaine, 793 in Gothia), etc.
The conquest of Gothia and the constitution of Marca Hispanica was an important strategical as well than ideological focus for Franks (unified on Peppinid/Carolingian rule) much more than Arabo-Andalusian had one in Gaul, giving a
relative disinterest and Abbassid pressure (that managed to ally themselves to Franks)
Maybe less by direct invasion (altough one was attempted in 763), than by proxy (using the rebellions that popped out in Al-Andalus or creating them as in 777), and by cutting off Muslim Spain from Arab reinforcements for a while, as did the Berber revolt for Berber reinforcements.
(A similar situation could be found as well with Fatimid pressure on Maghrib and support, even if half-hassed, of rebels as Ibn Hafsun)
Giving the limited forces avaible in Al-Andalus in the early times, priorities were drawn : by the end of VIIIth century, and the last important Arab reinforcement (Syrian
djunds, refugeeing from Africa), you had maybe around 15 000/20 000 Arabo-Berbers in Spain, whom the crushing majority, by far, was Berber.
(You could argue than a longer Umayyad Caliphate may allow a more important Arabic presence, but Spain was already well in the process of living its own by the late 730's. It would help, but maybe not that significantly)
It's at the point you had actual talks about abandoning Spain in the 720's.
And that's only one part of the issues : political concerns did probably as much if not more.
- Ethnic strife. It was probably more the case than any other place in the Arabo-Islamic world, with a really limited Arabic presence overnumbered by Berbers, native muladi and Christians.
While Arabisation remained being a way for social ascencion as everywhere else in the Dar al Islam, Arabity was what made it possible to be on top.
It's basically why Arabo-Andalusian nobility accepted to have Abd al Rahman I taking power : they didn't particularly loved Umayyads (they ignored most of what Damas said about how they should act) but the dynasty was seen as the champion of Arabity against converted peoples influences (It's telling that Cordouan court kept the same cultural and formal features than Damas').
Giving the limited Arab presence, preventing
Shu'ubiyya to be a thing there was a matter of political survival (if not simple survival). It did worked, but only delayed the issues.
This ethnical problem remained a thing for a large part of Arabo-Andalusian history (and certainly up to the Almoravid conquest at very least, afterwards it becomes more of a religious issue). It explains, for example, the muladi revolts in the Xth century (Ibn Marwan, Ibn Hafsun that went to the point to re-convert back to Christianism), and even it it went more or less resolved (for the "old" muladi mostly, newcomers weren't that well accepted. You did had Berber "pogroms" for instance during the Fitna) it greatly helped Christian kingdoms to strengthen.
Eventually, you even ended with ethnical-based taifas after the fall of the Caliphate : Arabs, Berbers, Saqaliba. While the Turkish and/or Mameluk takeover ended with clearly defined "foreign" dynasties, it never went at the point that the states they created were considered as such.
Taifas, on the other hand, while based on a common Arabo-Andalusian culture, remained distinct on this regard, less by cultural difference, than percieved differences.
- Political division. When I said "endless revolts" before, it wasn't really an exaggeration.
You'd argue fairly that Christians weren't exactly peaceful on the same period : but the ensembles remained relativly stable and the conception of a kingship needed to legitimize titles and power helped to preserve the post-Carolingian entities.
You simply didn't have such in Arabo-Andalusian Spain. Nobles and local rulers didn't felt compelled to preserve their ties to Caliphate that much for their own power : Banu Qasi dynasty is a good exemple on this regard, but hardly unique : they supported the appearance of Navarre as an allied Christian kingdom, considered themselves as the "third power in Spain", and forming a strong principality that fallen into different ones after their disappearance.
Don't get me wrong, you did have rulers managing to unify the Islamic part of the peninsula, such as Abd al-Rahman III or Almanzor on the late Cordoban period; and it didn't prevented the Caliphate to be a strong player in the region. But it remained a huge part of Al-Andalus instability, and explain the periods with collapse of central power (late
IXth, Fitna), and and unified order had to be maintained by force (usually meaning using mercenaries, see below)
- Not a clear border between Christian and Muslim Spain. While "border" in ancient and medieval era is a blurry concept at best, the "Marches" of Al-Andalus were some sort of huge buffer region (that could represent as much as 1/3 of the peninsula). You had regular exchanges hostiles (both sides raiding each other) but as well more peaceful : Christian emigrating in Al-Andalus (Neo-Mozarabs and mercenaries), mutual influence (Christians adopting Arab features, as well than the reverse : the military equipment of the average Arabo-Andalusian was very similar to the Northern Christian's)
It meant that the Christian conquests weren't made (in the Cordoban period, afterwards, the distinction is more important) on a totally foreign culture and population. It certainly made the firsts wave of Reconquista easier.
- Use of mercenaries. Giving the political desunity trend, you had a regular use of mercenaries. Berbers (newcomers, not really integrated into Andalusian society, and often wary/hostile when it came to civil society and more loyal to military chiefs) and even Christians, that weren't that much loyal either.
It's less by a lack of manpower (even if Al-Andalus may have known such issue, hence the immigration of northern Christians) than a political problem : cities militias, personal forces weren't that much tied up to Arabo-Andalusian emir/Caliphe (or their representants).
French and German kingdoms will unite in their effort to "liberate" the Iberian peninsulla?
That's of course, another factor. Less by blunt conquest from them (while the creation of the Marca Hispanica and the creation of several entities as Navarre, Aragon and Catalan counties stregnthened the presence of a Christian Spain).
(I would note that considering the afflux of
Hispanii, refugees from Al-Andalus, in Carolingian Spain and Southern France, you did have a genuine liberation feeling while not the only support for this campaign)
But more by the regular presence of reinforcement that Al-Andalus didn't beneficied at the same scale : from Aquitaine, Tolsan, Normandy (or other parts) on the military part, or on settlement (from Aquitaine and Provence mostly). It represented an huge part of the strengthening presence of Christian hegemon on conquered parts, especially in the East and Levante (peasants in XVth could often consider themselves as "Lemosin").
Navarre hegemon on Christian Spain in the early XIth is partially explained trough this.
Sorry if it's long (arguably, it could have been even more long, but I prefer to limit myself to Cordoban period, Berber dynasties as Almoravids and Almohads being another issue, based mostly on their instability and unability to face troubles both in Africa and in Spain; and the lack of real naval dominance on the other hand).
Each problem taken individually wasn't urremediable, but there's a lot of issues there, whom most are directly coming from the conditions of the conquest in first place.
It doesn't mean Muslim Spain was doomed to fall, but Al-Andalus as a distinct cultural and political construction (the distinction between both concepts is, IMHO, important) probably had too much issues based on its very existance to really be able to survive them (again, the main political stance on issue was to put them under the rug, and to delay them as much as humanly possible), encountering similar issues than Muslim Sicily, for example.
Basically :
- Not enough forces to maintain a continued expansion.
- Huge divisions based on ethnic/politic/dynastic/cultural feature
- Relying on a more merchant based economy (explaining partially troubles when silver production declined in the IXth century)
appeared since the creation of Al-Andalus (and were largely created by how the conquest happened) and prevented it to remain a peninsular power.