Sure, it does happen. The question is whether it happens frequently enough to satisfy OEMs desires for how often they want new API and how fast that API materializes.
And just having the source isn’t really enough. There are no StackOverflow articles on how to work with MS shared source code.
You’re thinking about best case scenarios, but markets move based on median case.
I don't why you think this is some pie in the sky fantasy. What I said is _already_ happening. This is normal stuff. For e.g. https://www.microsoft.com/en-us/sharedsource/
Or specifically - https://www.microsoft.com/en-us/sharedsource/windows-embedde...