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.
this is an over-correction of sensationalist narratives from earlier historiography. notions of apocalypse and decline are very present in late roman writing and meshed well with christian eschatology. Gibbon worked mostly with Roman written sources so it really pervaded his work---perhaps to an unreasonable extent---but the notion that things changed imperceptibly is also bunk in light of primary sources spelling that opinion out.
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Procopius, Zosimus, Gildas, John Lydus. The Fall of Rome and the End of Civilization is a relatively recent book that consciously catalogues the bad & ugly to synthesize with the current historiography