Can you provide some examples of languages where people regret having implemented generics at all (as opposed to wishing they had been implemented in a different way)?

Java is the obvious example that springs to mind. Because of the requirement to be backwards compatible with Java 1.4, all the type information is erased during compilation. The call for reified types in Java has been heard for over a decade now.

C++ compile type templating is the cause of egregiously long compilation speeds. One of the original drivers for the creation of Go was the speed of C++ compilation.

I don't think you can separate a desire that generics was implemented a different way in these examples from the regret that their current implementation causes.

If you are willing to pay with execution speed, you can get fast compiles for generics. But C++ opted for fast execution speed and hence pay with long compile times.

I'd much rather have fast compilation and slower execution and some kind of generics, but the current situation is that people are insanely attached to their pride in fast program execution.

This is a common argument, but too much software is painfully slow to use. It becomes a user experience problem.

I thought the lesson was "adding generics after the point where you have to care about backwards compatibility is fraught with peril".