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)?
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.
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.