After the fall of Rome, Venice was the doomsday bunker of the elites. Extremely defendable position (back when cannons didn't exist), close enough to the shore to transport things and people quickly, yet far enough that it's impossible to besiege. They pooled all their money to build a city on water while everything else around them was on fire both politically and literally.
Constantinople wasn't a community on a receding border, it was the heart of the empire. And it took centuries for it to fall definitely.
Constantinople is the historical equivalent of a receding hairline
not in the 6th century it wasn't. of course any history that lasts nearly 1000 years is full of ups and downs, and most ideas of extremely long term protracted decline are just wrong. but constantinople's decline starts slowly with the loss of egypt, and then becomes irreversible in 1204. the city was a hollow shell in 1453 and its the ottomans who build it as the center of an imperial administrative state. but if you're a rich roman magnate 800 years earlier and you wanna run away from the lombards, you ain't going to the swamp village close to ravenna.
in the context of the caliphate's eating of Roman territory the idea Constantinople was unsafe actually held a lot of water, with Constans II moving the capital to Syracuse and 10 years after his death the arabs very seriously threatened it in the famous 4 year siege