To add some nuance to the rest of the comments posted here: GPL’d code can be made proprietary, if the copyright holders all agree. For example, given a project that requires copyright assignment, if the project owner decides to take it proprietary, they can do that, because they’re the copyright owner. GPL alone is not enough to keep a codebase FLOSS. Luckily, both the kernel and git have hundreds of copyright owners (and does not require copyright assignment), so legally changing the license of either of them is practically impossible. So, really, Linus wouldn’t be able to take anything, code wise. He could take his future work, yeah, but he hasn’t been doing much development for the past decade.

Otherwise… He let go of git pretty early on, and it’s been maintained by Junio ever since. So nothing would happen there whatsoever, Linus’ retirement (friendly or otherwise) would be inconsequential for git.

The kernel has capable maintainers who have been maintaining stable trees for a long while now (often with companies backing them), and people who have been maintaining large subtrees. There’s a considerable overlap there, too. In short, there are a fair number of people who could fill in for Linus in a pinch. There’d be a small hiccup, and that’s about it. His skills and experience would be missed, but it wouldn’t cause any lasting harm.