The legacy systems need to upgrade because they need security patches. It’s not an incompatible system from 1992. Windows broke stuff going to Vista, when real-time controllers like you’d find running a power plant or CNC changed in ways I don’t remember, but fundamentally you could run a 35 year old application on windows 11 with tweaks over the years. These companies are running apps written in the 70s and 80s.

Microsoft could and perhaps should bifurcate Windows into new and old, or draw a line along Server and Workstation, but I think the bulk of their windows income comes from these enterprises and the “new” windows wouldn’t sell that well - it’s effectively been free for consumers since 8. Windows was the absolute show runner for decades but since the Cloud, it’s shrank quite a lot so there isn’t the money there anymore.

ARM is an interesting experiment they’ve been working on from a couple angles over the years but never really got the buy in.