The thing is, who's going to pull the cart? Giving stewardship to a nonprofit is perfectly fine from an IP perspective, and they'll have no trouble finding someone to herd some bugfixes, but what's going to happen when significant refactoring is needed to add support for C++26/29/etc?

Having your resident compiler expert upstream the occasional patch to keep your legacy product from dying is one thing, but which big tech company cares about this specific compiler enough to spin up an entirely new dedicated team for it? And who's going to choose a mostly-abandoned compiler front-end as a core part of a new product?

Sure, it can be done, but for each individual user defecting to clang must be looking very attractive right now.