Functions are a thing. Interfaces are a thing. I have literally never, in 6 years of full time go development, just copied and pasted code for the purpose of making it work with a different type. Nor has anyone I have worked with in that time (to my knowledge). I have seen two or three libraries written that use interface{}, but these are by far the exception.

Just depends on your application domain. I never copy/pasted code and could count the number of times I wrote `interface{}` on a single hand.