Office Open XML was only standardized in order to combat the threat posed by Open Document as organisations were starting to mandate use of standardized formats.
You write as if Microsoft did this because they wanted interoperability, when in reality they only begrudgingly accept that some must be allowed in order to avoid losing control of the market.
The real solution would have been to never approve the OOXML standard and not legitimize Microsoft's attempt to make their proprietary format appear open.
My old mother, who is completely disinterested in technology, has used a Linux desktop for a decade now without major issues.
If you aren't a power user the differences between it and Windows are minor. You have windows, icons, menu bars, x closes the application, the box makes it big, right-click to open a menu, left-click to select, it's all the same stuff. Besides, most of your time is spend in a browser anyway.
Yeah things break some times, but no more than in Windows. Being on a very default Ubuntu installation she can just search for her problems online and blindly run some random console command that probably fixes it, just like on Windows.
Hardware is easier because drivers are generally just magically there. Software is easier because it's mostly in a repository which automatically installs dependencies and updates and doesn't come with malware.
By far the biggest problem has been documents and executables that can only be opened in Windows. Mostly PDF forms (fuck you Adobe).