There is a terminology issue here, with "generic" being overloaded to mean different things. Parametric polymorphism is what Go is adding. You'd like to see something like ad hoc polymorphism in Elm, like type-classes in Haskell. But neither of those should be confused with the datatype generic programming.

https://en.wikipedia.org/wiki/Parametric_polymorphism

https://en.wikipedia.org/wiki/Ad_hoc_polymorphism

https://wiki.haskell.org/Generics