OK, so the problem is that the Roman roads aren't being maintained due to fragmentation.
What made it so that the medieval kingdoms didn't maintain the Roman roads internal to their countries? Lack of funds or manpower? Lack of cities? Feudalism?
No, rather they weren't been maintained due to the decline of the roman state structure and administration, between the VIIth and IXth centuries.
We know Merovingians and Carolingians regularily acted to maintain Roman roads as much they could, as part of the public fisc. Not all roads, and it was essentially about limitating their disuse, but Roman roads up to the Xth century are the basic infrastructures for transportation (civilian or military) as point both the mentions of roads (British "streets", "Chausée Brunehaut" etc.) and a quick glance at military campaign and battles maps.
Anecdotically, we know that camels were used as oxen-driven chariots in Merovingian Gaul for transportation, and it's likely it was the same in Spain and Italy.
Several things changed with the Late Carolingian period.
- The collapse of the romanised state and what we could call the feudal anarchy up to the XIth century. Not that you didn't have a road network, but it was heavily decentralized, not that used (at least until the early XIth, land exchanges were limited and short-range) and "moving", so to speak, mostly dirt paths.
- The final decline of the administrative bureaucracy and the triumph of feudality (which is something Carolingians not only supported originally, but more or less provoked in their bid for power) meant that the fiscal revenues needed for a costly road network were extremely divided and no longer monetarized until the XIIth century. At best you could resort to
corvée for public great works as French state did between the XVIth and XVIIIth century, but the desintegration of the state authority prevented this to happen on a regional scale.
- Technological development : while generally caricatured to the point of absurd unefficience, the horse collar used so far was still less practical than the breast collar that began to get widespread at this period, in the same time than stirrups. It made transportation and communication less dependent on heavy-maintenance road network and rather fit the new situation.
- Large use of rivers/sea/lakes/etc. : a bit like in pre-Roman times, transportation on ships was really dominant. In a "Roads? Where we're going, we don't need roads."-way. It was relatively efficient, cheap and quick enough up to the point cities more or less dug artificial canals when they could for intra-urban transportation.
How about if the Muslims in North Africa started maintence attempts on the Roman roads?
Medieval Arabo-Islamic civilization is litterally the last ensemble you'd want to restore a road network. To quote Richard Bulliet, it was a wheel-less civilisation, with an earlier stress on horses than in medieval world, few use of wagons, important use of camels in Africa, etc. Note that it wasn't all the same everywhere and al-Andalus kept a strong network.
But, generally, medieval Arabo-Islamic world didn't really cared for a road nework that would be close to what Romans had.
Also suppose East Francia, Lothringia, Barvaria (which at one point was de facto independent), the Papal States, whichever power controls Northern Italy (I think the Kingdom of Italy still existed?), Wessex, Mercia, navarre and Castile also attempted their own maintence attempts?
Roman road network was expensive. I mean REALLY expensive, both to build and even more to maintain.
Assuming no technological development whatsoever as described above, imagining that these ensembles doesn't crumble the same way than anyone else (Italy specifically was a feudal desintegration on steroids, but
@Carp would be best to speak about it), how can they fund these while still managing to pull the basic expense of any feudal entity, meaning productive redistribution and warring.
as marriages lead to personal unions upon death of senior heirs
It sounds nice, but works a bit too much like a Crusader Kings game to be that believable : again, the key feature there is that between the late IXth and middle XIth, most of western europe is basically a decentralized ensemble. More unified in Germany, England and northern France almost litterally atomized in Aquitaine, Provence and Italy.
As for personal union, when Henri II inherited the lot of western French and Aquitain principalties, he didn't made him undisputed ruler of these lands and able to drain its wealth for great works, but more or less a feudal hegemon that had to deal as much with his own rebellious and independently-minded nobility than with the foes outside his domains.
Furthermore, and this is really important : roman road network was made for a geopolitical giant. The need for quick communication and transportation was a matter of imperial interest, if not survival (both for military and fiscal reasons). It was not so much for western Europe in the XIth (for reasons aformentioned) and even Byzzies mostly used the Via Egnatia for military purposes rather than economical. (Truth to be told, even roads in Byzantine Empire were more often than not in disrepair).
Add in "it's running well enough now that we don't need to invest" ideals of politicians too that caused
modern day networks to fall behind.
Well, as bureaucratic states made a comeback since the XIIth century, one of the first things that they tried to settle was the matter of roads. You have some royal ordinances in France about the management of roads, but it remains in the spirit of late feudal bureaucratic states, with nomination of officials in charge of precise portions (not unlike private highway companies, mind you, except on a smaller scale) and its "nationalisation" happened significantly later (possibly delayed by the crisis of the Late Middle Ages). Elsewhere it was a matter of municipal management or semi-private great works (especially for bridges)
A public road network in the western world really begins to appear only in the XVth, and definitely in the early XVIIth.