As others are alluding to in the comments, I’m highly skeptical of coupling the issue tracker and other non-VCS features with git. A far better solution would be to keep them decoupled, but easily pluggable and extendable.
In fact, this is basically the status quo with any issue tracker that has git integration (ability to tag issues in commits, etc.) If this was the only problem GitHub solved, then it would be competing with pivotal and the like.
But issue tracking is not the only problem GitHub solves. The reason GitHub grew so large is because it created a community within a single space. It gave devs a place to collaborate on, and discover projects within an environment highly integrated with VCS and issue tracking.
People don’t publish to GitHub for the issue tracker or repo hosting. Those are solved problems and many companies even use separate software for them. People publish to GitHub for the visibility, discoverability, and community.
There is no technical solution that can usurp the advantages of GitHub as a social network, just like creating mastodon is not enough to kill twitter.
The technology features of GitHub are easily substitutable. The community is not. Network effects are real, and they are the reason Microsoft felt confident paying $7.5bn for a platform that has no major technical differentiation from its competitors, but does have a huge and defensible moat around mindshare, due to its main value-add being rooted in network effects.