Are you seriously saying that github.com/somelib is a better package management? Of course nobody forces to use that, but it's what actually been used in most go projects. Even NPM looks better in comparison.
The best quality package management I've seen is Maven, followed by Cargo.
This doesn't mean people _will_ name their things sensibly of course. I've seen golang projects with packages named after gardening, meteorology, and aviation… and each is totally confusing when you're new to it, especially when the naming theme is completely unrelated to what the project actually does.
Here maven would be acceptable (similar to go, there is a domain identifier embedded) whereas cargo (bare words) wouldn't be great.
for private Go packages all I need is to point it to another git repo. for nuget/maven etc - I need an artifact registry, with it's own bells and whistles
Well, yeah, I mean if one can say Maven is best package management then I think I can get away by saying Earth is flat or stationary or some such interesting facts.
Github repos can vanish, NPM will only let repos be deleted if they have no direct dependencies also on NPM. So you have some guarantees your dependencies wont vanish.
You can also configure go to use your own for internal projects or if you just want to avoid the global proxy.