The problem isn't primarily "tourists who are not spending enough". It's too many tourists in general to the point that the city infrastructure can't actually support that many people and is overcrowded making it horrible for anyone who is trying to actually live there.
Another point is that overcrowding also makes it less pleasant for the tourists themselves. If you charge people €10 and they find it more pleasant, don't have to queue for drinks or to take selfies, and can get help if they need it, they might consider it well worth it.
I wonder what percentage of tourists that visit Venice would see it as "bucket list" or "once in a lifetime" type opportunity to the point that even a €500 entry ticket wouldn't make a noticeable dint in numbers. A lottery system seems a better (and fairer) bet, and those that miss out would just have to try their luck finding accommodation (which actually wasn't that hard when we tried 11 years ago, and personally I don't believe you can meaningfully take in a city like Venice in a single day - or even two for that matter).
