This proposal is simply to retain the existing GOPATH functionality, along with the go.mod modules.

The rationale is that GOPATH provides an ideal mode of operation for developing across multiple different repositories at the same time. It is otherwise cumbersome to do so with go.mod, requiring e.g., temporary insertion and removal of replace directives (see discussion in #26640).

A major design priority for Go is simplicity, and GOPATH is simple. It provides a great way for new users to explore lots of different code packages easily, browse, modify, experiment, etc.

There are many different types of Go users, but it would be great to retain support for the "hobbyist" / non-corporate hacker who just wants to do things simply and doesn't care about living on the bleeding edge.

It should not be so hard for the two modes to coexist, as the tooling etc is currently supporting both, and I personally find it most productive to develop in GOPATH mode and then have some Makefile targets that ensure go.mod still works for those who want to use that. And all official releases are done in go.mod.

Thus, it may actually be optimal to have both modes available, instead of trying to complicate go.mod so that it does what GOPATH already does so well.