If Microsoft backs off core Windows development, which it certainly seems like they are, this is what we'd expect to see... not a big bang where it suddenly stops working, but every year it'll just sort of fall a bit more behind Linux, until in 4 or 5 years the difference consensus will slowly just creep over to Windows being noticeably worse than Linux without any big bang switchover... and it'll sneak up on Microso…
Interesting thought, but you might be missing the hardware perspective. I switched from Windows to Linux this year, and the biggest issue has been drivers and hardware compatibility: audio available via headphones but not (laptop) built-in speakers, occasional display issues depending on the monitor setup, occasional WiFi issues. If hardware innovation stopped, Linux could catch up easily... but it won't, so there is…
I do believe that manufacturers would be willing to help out if GNU/Linux was sufficiently standardized that they can simply drop some code/binary that works on basically every GNU/Linux distribution (just as they can basically do on Windows). The problem is that GNU/Linux is not some single operating system, but a proliferation of lots of different distributions.