"But instead of defining an ARM based PC"
The reason is the walled garden which worked so well for iOS made MS greed and hungry. They exactly didn't want to replicate the PC business where IBM first, and later MS almost too became irrelevant. Really, MS doesn't care about hardware you can run Linux, Solaris, FreeBSD, etc. on. And why should it? No revenues from it. MS is a business company, not a charity. Does Google help to build alternative search engines?
The PC was an exception, not the rule. IBM didn't really believe in it, and so didn't protect it. It was a lucky accident. If it could get back in the past, IBM would surely fix such a mistake.
Yet, sorry, governments - but authoritarian ones - can't intervene and "force the market open". It would be a violation of basic democratic rights.