[stubish]

Other languages don't allow you to import multiple versions of a package. Allowing this opens a can of worms, mostly to do with global state. The two versions of the module can still be competing for the same global resources, say lock files, or registering themselves somewhere, or just standard library stuff like the flag package. Unless developers actually test all their major versions running together, you are just crossing your fingers and hoping. It was the same problems we had with vendoring, and one of the reasons the 'libraries should not vendor' recommendation is made.