It occurs to me that, during the initial expansionary phase of Islam, the importance of Constantinople within the Byzantine Empire became proportionately greater, as more and more of the Empire's territory slipped away, with its major urban rivals falling under foreign rule (antioch, alexandria).
There were certainly many years where the Empire was Constantinople (not in the literal sense, of course). Consider that most of history considered city-states to be the best polities for Republican rule. Add into this the fact that the Byzantines had quite a few Emperors who, while not necessarily bad, weren't quite up to he challenges that faced them. For another added wrinkle, it seems that this period really saw the Byzantines seriously questioning their worldview, due to the obvious calamaties they faced.
With all of this going on, is it possible for the elite to decide that, hey, maybe going back to a Republic is a good idea?
Rhetorically, they could point out that the Empire did almost all of its expanding as a Republic, and all of its losses came as a monarchy (rhetoric allows you to play fast and loose with the facts). They could also allude to some Greek heritage of democratic rule, if they want to get really classical.
Strategically, if these elite take Constantinople, they could lose pretty mich thr rest of the Empire, and still be in a good position. Even if they can't hold onto much of the territory, they could be super-Venice.
Thoughts?