The fact that they needed to make a map a part of the language in order to allow it to be generic and statically-typed proves that generics are useful and should therefore have been a language feature much earlier than they became one.
There are a variety of things that the standard library or compiler deal with using weird workarounds that seem to indicate missing language features.
The thing is, the features are only "missing" if the language is designed to do the things those features permit. So the counterargument is that Go is a very opinionated language designed to do solve a few classes of problem very easily, like writing database-backed web services, and the reason the standard library or compiler teams have to do weird hacks at times is because Go wasn't made for writing those things, and designing to make those use cases easy would pollute the language from the perspective of someone using it for its intended purpose.