A change in your repo does not need to result in an import path change if you set up vanity URLs for your import path. For instance, all my go libraries use the import path of gomod.garykim.dev even though the repos are actually hosted on GitHub. That means if I ever change to self-hosting, for example, I just need to update some of the meta tags on the gomod.garykim.dev website to change the repo location.

That still means that if you lose your domain name (gomod.garykim.dev) for whatever reason, you have to modify your code.

You can use a replace directive to handle cases like this without updating the code.

https://golang.org/ref/mod#go-mod-file-replace

Only if you're the final consumer of that module - if you publish it so others use your module, the replace directive will be ignored and they will get a failed build (or have to add their own replace directive for your sub-dependency).

Yes, but this is roughly the same as any packaging system - the ones that don't use DNS as a default namespace require you to add explicit package repositories, which is no more or less onerous than the replace directive.