All 3 of the big OSes have extreme amounts of business critical code from the 90s and 80s. Code that is complete spaghetti, with no automated tests, no peer reviews, no changes for decades, and written in C of course (objective C in case of apple)
That alone is a reason to start over with a from-scratch mes desktop OS implementation
Microsoft and Apple both have more than enough money to do it. It would be a ~10 year project. It would not make money which is why it’ll never be done.