The team I work in use generics all the time and I'm not sure what complexity you are referring to. Care to elaborate? Is it some edge cases or are you talking about from a compiler perspective or something else?
To me, not having generics is like saying let's skip handling bools and just store them in strings as "true" or "false". It's such a weird thing from my point of view.
Note that, for all of that, I'm still in favor of adding generics in Go, as I do see the value they can add. I'm just pointing out that "bashing" on Go devs on not knowing what generics are or how they can be useful is both non-productive and probably generally wrong. Plenty of us are saying "Man, if I had generics, this would have been less code/cleaner/more reusable!", just the other various tradeoffs outweigh the actual cost of using a different language. Go's ability to drop a very junior dev (or just one that has never use Go) into a codebase and have them be almost immediately productive is incredible.
Interfaces are boxed already, so you've paid the price without having type safety. Simply treat a polymorphic function as if it accepted interface{}.
(I also think it makes sense for TypeScript given JavaScript idioms; it mainly falls over when the caller has to interpret some very complicated error messages, however)