There's an ambiguity in the phrase "then go.mod wouldn't have existed." One way to read it is "then, as a consequence, go.mod wouldn't have existed" and the other is "then go.mod wouldn't have existed at that time." I believe the intention was the latter, whereas you're inferring the former.

go.mod provides a way to introduce the new behaviour incrementally. The key point is the dependencies can (and already do) declare different go versions in their go.mod file. When everything is compiled, each module is compiled with the behaviour that applies to its own declared go version. So even if you update your go.mod go version to take advantage of this (soon to be) new behaviour, you can continue to use existing deps happily without worrying they will break.

Because of time? How would you use go.mod when go mod does not exist?