Well, in the middle ages, Venice had no objection to fixing flooding by piling a few more layers of stone and wood onto an existing (sunken) structure and using it as a foundation. And a few hundred years later, doing it again. It didn't used to be a static city.

But now that the structures are considered historical and preserved, they're trying to maintain a particular snapshot of the city, with the exact same buildings. Which is a lot harder than maintaining the city overall.