Microsoft has no incentive to do that unless they care about the success of ARM as a PC platform. Why should they? ARM doesn't bring anything to the table they don't have with x86, and they already have a competitive market for x86 now that AMD has put the Bulldozer days behind it. Any increase in ARM PC sales is essentially a decrease in x86 PC sales, it won't expand the market for Windows. Why should Microsoft create friction with OEMs over something that doesn't make them more money?
Microsoft would get a lot of complaints from OEMs if they tried what you say, because that it will increase their cost without zero prospect overall sales will increase for the same reason it won't increase Windows sales.
Maybe OEMs decide to tell Microsoft "OK we're done updating our drivers for all our existing products to avoid having to port them to ARM, and if you make changes to Windows that break them, when customers call us we will be blaming you!"
Ignoring the driver comment this is replying to but addressing the independent claims in this:
Any increase in ARM PC sales is essentially a decrease in x86 PC sales, it won't expand the market for Windows. Why should Microsoft create friction with OEMs over something that doesn't make them more money?
You think MS is happy about Apple's creeping marketshare of premium with the M-series? They have to have something in thin and light that can seriously compete with those SoC's on idle and "real-world" (read: light use interested by heavy use and/or background tasks) use and an increasing proportion of both activity and sales are brought by mobile AKA laptops, not desktop DIY nerd stuff.
I want to highlight something: MS wants the most competitive Windows device market possible, because an uncompetitive one - by virtue of the players and their capabilities (see Intel, AMD uncore fabric and idle stuff) number of players, etc is one that loses premium share to Apple and/or puts Windows in decline for key segments. Competition is not an on/off switch. I think it's very clear they don't just stop and say "well AMD and Intel are doing ok" certainly not after watching the last 20 years.
MS also has Azure and servers as I pointed out elsewhere; as such, I suspect they have some Neoverse-esque design cooking in the pipeline and ergo have a vested interest in Windows being *in principle* first class on Arm, and that’s going to have crossover between the server and desktop. Delusional to think otherwise. It also reduces the marginal cost of MS maintaining e.g. VS and the kernel for Arm etc.
You're confused about "expanding the market for Windows" vs keeping Windows competitive and staving off any possible decline due to a lack of appealing hardware or otherwise. It's true in OP's fiction that it might create friction, but that's not what they're actually doing RE: Arm. They don't have to create any unnecessary friction. They work with Qc, Nvidia, Samsung/MediaTek, port the core Windows structures - which they have been doing in fifth gear in the last year - and let it rip.
More Windows competition and options = good for MS. Not hard. Duopoly is much riskier from their vantage point. And before someone high as a kite says it, no, AMD and Intel do not have a magical Linux alternative and ChromeOS will continue to be X86/Arm both.