That's a completely silly response, but I'll give you points for at least making me wonder how they collect trash in Venice. Turns out that:

a) it's a huge problem in the canal district (thus making my point)

b) they have boats with trash compactors:

https://www.c40.org/case-studies/new-waste-collection-system...

My point is these problems are solvable if you make an actual effort to solve them. There are costs and tradeoffs, but it's crazy to just throw up your hands and assume a city without roads is impossible as soon as you think of a problem rather than taking 5 minutes to actually try to address it.