Thanks. I was hoping you were going to fill in on the divisions in the early medieval west, since my gaze is firmly fixed between Constantinople and Damascus, and thus I don't get much of a chance to look west very often.
I do suspect (but haven't looked into the most important recent work on the topic, a book in French by Christian Settipani) that the final death-blow to the senatorials in the Byzantine lands was less the loss of the city and moreso their complete loss of function. Caucasian aristocrats seem to have eagerly stepped into military roles in Byzantium in the seventh and eighth centuries, creating a new aristocracy that got its power from the army and was not based in Graeco-Roman culture. I would also hazard a guess that this is related to the lack of literature in classical genres from the "dark age" period; these new aristocrats were largely busy elsewhere, and on top of that there was no good story with a beginning, middle, and end to tell, since Byzantium was still fighting for its life. Throughout the entire Roman the elites had served as agents in the collection of taxes, but this seems to have become particularly stringent under the Dominate. With the collapse of the standing army in the seventh century and the subsequent economic damage, specially appointed imperial officials appear and seem to have become the IRS in the countryside, further diminishing the already limited roles of the traditional elites.
Indeed. The case of Gaul around the same time period I believe to be somewhat a similar transformation, although Gaul of this period is not my specialty. I do want to stress the similarity, that instead of what we had going on in Iberia- IE an almost complete absorbtion of the Goths into the Senatorial Aristocracy, and assuming many aspects of their administrative and poltiical functionalities- the Frankish realm, perhaps due to holding equal weight across the Rhine, continued down the path of replacing the Senatorial elite with a new, militarized aristocracy, entirely Germanic in it's
capabilities- but, ironically,
Latin speaking and mostly indigenous. The process is similar to what happened in Anatolia, albeit obviously without the influence of Constantinople direct control, and the lack of the Germanic (IE like Byzantium's Armenian and Caucasian) families importing themselves; it was essentially their practical roles that were adopted. It is also, however, in the west, a continuation of the events of the 5th century- but developing opposite after the 6th century to that of Gothic hispania.
Kind of a mouthful,

but the process is indeed hard to explain. It's also important that we give Frankish lands across the Rhine their bit of influence on these developments, too. Similarly to how the Armenian and Caucasian influences began to really take effect in the 7th century in Byzantium, so too did the
Germanic, non-Frankish influences from beyond the Rhine take their hold as well. Previously the caste of the Franks followed the path of the Goths, but after the 6th century, their roles and assimilation into gaul were interrupted and began to show influence of
Germania- as did the whole of Francia north of Provence, Septimania and Aquitaine.
Very interesting the whole tax system was in Late Antiquity. Similarly to both Byzantium, and surprisingly,
Lombardy, the tax systems broke down in a very similar manner.