I think they're removing old rust that would crate friction in moving towards AI assisted development. Old rust which is used to higher quality of code...
Though based on the headlines that I've seen regarding Azure development I'm not confident that they would end up in a better place if this ends up happening.
But maybe who knows, they might open source the NT kernel as there will be nobody left to maintain it within Microsoft.
NT Kernel, Direct3D, .NET Runtime. Also a lot of stuff that came out of Microsoft Research like Z3.
Which also happen to be the sort of projects older devs would normally be working on.