Pirenne, illustrated

Here is Trade And Antiquity (pdf): page 14 has been linked, here.

If you wanted "what happens when the whole of Anatolia sinks into the ocean", that's pretty much what we got. Late Antiquity, once more, applied to when Constantine's City is truly the Roman Empire, everything else revolving around it. After that, it's all islands in a hostile sea. Byzantium still exists, yes. But so does Spain. They're about as useful as each other. Somewhere below there are Sicily and Morocco, maybe Tunis and some sites in north Italy. Rome is, basically, nowhere.

What we get instead, in the time of the caliphs, is two rival island continents. One is the continent of Syria; the other is the continent of... Neustria-Austrasia a.k.a. northern France.

This still looks like a Dark Age to me. Unless you were lucky enough to be a Syrian, or attached to the Carolingian court.