There's another realistic possibility. A human-played third power like Venice, Austria Muscowy or even the Memelukes could want to try to keep it alive to weaken the Ottomans, and could manage to make that happen by vassalizing and then defending Byzantium. It makes the most sense for Venice, I think, though keeping the Ottomans at bay would not be easy for them. Austria and Muscowy are a bit far. Mamelukes would do it easily but its uncertain if they'd stand much to gain from it.
Possibly but as Venice i'd likely more want to control Constantinople than have it as a vassal. And as the NPC capital of hte BYZ you would have to take their holdings in greece as well to do that. Territorial control of a trade node is important for trade oriented nations like Venice.