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.
venice was barely a city when the exarch was punted from ravenna. literally a few villages of Swamp People the romans sent a few titles to the elected leader of--and in those days they were poor and egalitarian, a constantinopolitan noble would've found it positively gauche. also this is 2 centuries after the 'world ended' in Italy, under Justinian's Gothic War & plague
frankly i'd move to Sardinia if peaceful retirement was the idea, that place so steadily faded from roman rule its de facto feudal lords were roman judge offices