> they would need to reinvent the whole UI stack of Linux like MacOS did.
MacOS X is not based on Linux or GNU and it's UI evolved out of a combination of Next technology and features combined with re-implemented classic Mac OS features. So it wasn't exactly a clean slate.
More relevantly though, they put some effort into maintaining compatibility with programs written for their previous OS by both porting their APIs and providing CPU emulation! And that is on top of bringing the rest of nextstep up to standard for what consumers were expecting out of a desktop OS at the time.
> Keeping a 50 people hired for Kernel development sounds cheaper.
I doubt it, Microsoft can and has rewritten their UI multiple times... if they wanted to, they could write it for Linux atop their own ported windows API with little effort. I think they just lack the incentive to do so, Linux users wont care because they wont use it, Windows users wont care because they don't care about Linux... who will benefit? not Microsoft that's for sure.
From a business perspective MS behavior makes sense, they are embracing Linux because that's the way one half of the market is headed whether they like it or not... meanwhile their desktop business still has a hold on the market mostly out of inertia and legacy, so why would they do something as radical as change the core OS to mess that up?
However as the parent said "one day", maybe, in the future, when windows makes zero sense to run on metal, then perhaps microsoft will create an official and FOSS windows API... if only to relieve themselves from security support hell.