Not everything is always a slippery slope argument…Also good luck enforcing this in any other city. This only works in venice because there are only a few ways to get in. To do that in lets say nice you would need to fortify the city…That will just not happen.
Just don’t visit in the main months. I was on San torini this year on April and it was super nice without so many people. Yes you couldn’t swim because it was too cold but that’s it. The experience otherwise is the same