Arabic by contrast is arguably still one mutually intelligible language
In the spoken variety, it definitely is not.
Educated Moroccans and Iraqis may understand each other speaking Standard Arabic, which they would have learned at school and absorbed through the media, but in general they would not be able to do that in their respective everyday spoken languages, which are about as divergent as Romance languages if not more.
The religious and literary prestige of the Classical language and its Modern Standard derivation keeps the Arab World linguistically unified at the written and media level, but actual spoken language is different. This is somewhat changing as the Standard, with regional differences and somewhat of an Egyptian tinge, becomes more widespread thanks to modern media and increasingly widespread schooling, but still largely the case.
Educated people often mix varietes, and have been doing so for a long time.
Also, there's evidence pointing to spoken Arabic varieties diverging pretty quick, as as early as the tenth century Arab Grammarians were able to point out varieties in some areas. Scholars are not really in agreement on how to explain what happened, since we really lack vernacular written texts from early times outside some small exceptions, but there's increasing evidence pointing to "Arabic" having been a cluster of slightly different variaties as early as we can document it. Classical Arabic (and therefore Standard Arabic) would then be just the variety picked as the "official" one, in which the Qur'an was recited (it might be even more complicated than that, but nevermind) but it is possible that spoken Arabic in some places derives from variaties that have have diverged in pre-Islamic times.
Even the scant pre-Islamic evidence we have suggests huge dialectal difference within Arabic at the time.
Anyway, it is very likely that the rapid course of the Conquests led to a relatively quick linguistic replacement and change, as we find to be the case in Egypt for example, where Coptic probably turned into a minority language in about two centuries just to disappear sometime between the fourteenth and the seventeenth centuries (as the relatively low number of Coptic loanwords into spoken Egyptian Arabic would confirm; a longer period of widespread bilingualism would have led to a larger Coptic imprint). It seems that many people picked up Arabic, but at least at first, learned it imperfectly and this may have led to a partial "creolization" of the later spoken varieties. This model is supported by a prominent scholar in the field at least, but it is not universally accepted.