The non-adaptation thing is recent. Dig down and you will find every building today stands atop the rubble of many previous that were nocked down once they got too expensive to maintain. The city constantly sinks and readily adapted to this. Some might say that need to constantly adapt made venice great. But at some point in the 1950s/60s the "greatest generation" decided to protect the old at all costs. Nothing is ever knocked down and so within only a couple generations everything is in crisis at once.