I suppose it wouldn't. But, if you're going to do it as a feature that's built into the core language, I'd say that's even more reason to just do it Kotlin-style. The other main downside of doing it that way is that it needs to be baked into the core language and can't be pushed out to the standard library the way you can with optional types. But that wouldn't really be a practical difference compared to an optional type implemented without full-fledged generics.

The other big distinction between the Kotlin approach and optional types is that the Kotlin approach introduces no new run-time types. Null safety is checked statically, and then you're done. That's a big part of why it plays nicer with an existing standard library. It also means, though, that you introduce no extra run-time overhead, which I would assume is something that's considered pretty desirable to gophers.