Git shortcomings are well known by this point, so "all" a successor project has to do is solve those problems. Git scales to Linux kernel sized projects, but it turns out there are bigger, even more complex projects out there, so it doesn't scale to Google-sized organizations. You would want to support centralized and decentralized operation, but be aware of both, so it would support multiple remotes, while making it easier to keep them straight. Is the copy on Github up to date with gitlab, the CI system, and my laptop and my desktop? It would have to handle binaries well, and natively, so I can check-in my 100 MiB jpeg and not stuff things up. You'd want to use it both as a monorepo and as multirepos, by allowing you to checkout just a subtree of the monorepo. Locally, the workflow would need to both support git's complexity, while also being easier to use than git.
Anyway, those are the four things you'd have to hit in order to replace git, as I see them.
If you had such a system, getting people off git wouldn't be the issue - offer git compatibility and if they don't want to use the advanced features, they can just keep using their existing workflow with git. The problem with that though, is that then why use your new system.
Which gets to the point of, how do you make this exist as a global worldwide product? FAANG-sized companies have their own internal tools team to manage source code. Anywhere smaller doesn't have the budget to create such a thing from scratch but
You can't go off and make this product and then sell it to someone because how many companies are gonna go with an unproven new workflow tool that their engineers want? What's the TAM of companies for whom "git's not good enough", and have large enough pocketbooks?