As someone noticed earlier the introduction of a ticket to access a city (be it Venice or Florence) will probably only make a dent in the total amount of people visiting it (which is the actual problem, they are simply too many) while making a difference between the "wealthy" and the "poor".

If the issue is too many people, the only solution is to limit the number of people (within reason and using some common sense in the way these hypothetical limits are set and enforced), introducing a ticket won't make any difference if not - possibly - provide the municipalities with money to cover the expenses these tourists create (the first thing that comes to my mind is litter, but there is also water, street and bridges maintenance, etc.).