Github repos are namespaced to their owners, at least, radically reducing the potential for this kind of thing.
2. /s/github/whatever . As long as there's a public URL from which something can be cloned, the idea still works, as long as that URL doesn't someday point to something completely different. Again, not impossible, but less likely than with NPM.
3. There are, like it or not, real advantages to centralised archives - discoverability, optimisation of dependency resolution, etc. 'Privately controlled' is an elastic idea - it seems to me that there's a difference between a non-profit foundation, say, and a for-profit company like NPM or GH, but both are 'privately controlled'. The question is whether these advantages outweigh the disadvantages. In my opinion, they do.
It's almost as if privately controlled, centralized archives are a bad idea.