That's not true at all. A Go-modules-enabled project can import packages from a non-modules-enabled project that doesn't have Go.mod in it. This was always the case with Glide and Dep, too.
That's not what I'm talking about. I'm talking about being able to build outside of GOPATH. This requires the top-level project to use go.mod and many don't (like Docker for instance).
But that's not the same thing. You don't have to switch all the projects you work on over to Go modules at the same time, only the projects you want to upgrade. It's entirely possible to migrate incrementally.