Since this topic has never been brought up before I thought I'd be the first. I seem to increasingly come across travel columns where locals are beginning to rebel against cruise ships and overtourism in general.
In this case Venice is the focus of attention: https://www.cnn.com/travel/article/overtourism-cruise-industry/index.html
This website is interesting. https://avoid-crowds.com/london/?date=2019-10-06
On a scale of 100 London receives a score of 97 percent for an August weekend--Whoa, Nelly!--while a non-holiday in October is only a 10. (No offense intended to anyone named Nelly.) We were in London May 24, a 65 and a holiday (ugh), so I wonder what a 97 is like in comparison.
To the extent I can I try to avoid holidays and when the kids are out of school, but I can't always manage that. The last trip or so has really spurred me to consider the off season. Some day!