Tourism wasn't a big thing until the 20th century. Most people couldn't afford to travel.The term "Italians" means post 1866, I suppose ...
Making a polder would alleviate the flooding and thus preservation concerns. But might impact the tourism industry. The city's selling point is that it is VENICE, i.e. the city with canals instead of streets. The marketing spin would have to change.
A surge barrier like that in Sankt Peterburg would be better.
A polder which retains water around the city - and in the city itself - would be an eye sore. Again bad for tourism.
Tourism wasn't a big thing until the 20th century. Most people couldn't afford to travel.