And interrupt their mostly nontechnical employees’ productivity to learn a new OS, for a negligible cost savings? Good luck getting that past a board of directors.
For Microsoft? Windows has been on decline in importance at Microsoft for some time.
For Windows? Nah, too many Fortune 500 businesses rely on software running on Windows for day to day operations. While it's hard to see from the consumer/ web/ developer side of the world, Win32 still dominates. Lots of vertical industry specific software which won't be replaced for quite some time.