Except that you lose type safety (or write an implementation for every permutation of types..)

Didn't realize that, thanks.

Go doesn't have parametric polymorphism. So yeah, there is something stopping you.

Nope, not stopping you: http://blog.burntsushi.net/type-parametric-functions-golang

Yes, stopping me. "You can resort to a horrible ugly cludge and lose type safety for your efforts" is not having parametric polymorphism.