Everyone here will probably say people are moving to Linux, but I imagine the biggest beneficiaries of Windows’ long-term decline in market share are ChromeOS and MacOS. I would probably recommend a MacBook to my mother if she asked for my opinion on a new laptop.
There is also the argument that Windows’ market dominance was an aberration and unsustainable, and now we are seeing a reversion to the mean. While Windows is in an enshittification phase right now I wouldn’t be surprised if Microsoft eases off in a few years if their market share continues to decline.
This is good news, hopefully more competition in the OS space.