To go to the original question - initial disclaimer: I have no Apple or Microsoft inside knowledge, but I *WAS* at a hardware company in '99-2000 and again in 2008-2010, and dealt tangentially with the relationship between that vendor and Microsoft - including during the introduction of a new processor architecture. The below is largely conjecture, but it is "educated conjecture" based on similar experience.
- Apple has disclaimed ownership of this process. They have said it's up to Microsoft.
- Microsoft generally helps hardware vendors, but doesn't do all the work themselves.
- Microsoft has already done the major work needed in this instance, porting to ARM.
- Apple's ARM (as every ARM CPU) has custom instructions, and custom support chips. While "basic support" can almost certainly be written without inside knowledge, performance would not be good.
- This isn't even getting in to drivers for non-CPU components. This is purely "minimum bootable."
- Hardware vendors are generally 100% responsible for writing drivers for their hardware. Microsoft offers generous assistance in this process, but this process is completely owned by the hardware vendor.
- This means things like Apple's GPU, storage system, and many other *CORE* components, wouldn't work out of the box with just Microsoft developing. Especially since Apple won't release the hardware details.
- This means that Apple's "it's all on Microsoft" is at least partly a bluff. Apple hates poor user experience. (In their mind.) If Microsoft were to release a "retail Windows 10 for Apple Silicon Macs" with zero Apple involvement, it would *SUCK*. While some might think it would reflect badly on Microsoft, it would also reflect badly on Apple.
Apple wouldn't want that. Microsoft knows it.
- If Microsoft develops Windows for M1, they will let Apple know privately, and give Apple the chance to write drivers.
- Apple isn't against writing Windows software/drivers - Boot Camp has existed since early on in the Intel timeframe, iTunes for Windows has existed longer than that, QuickTime for Windows since the early '90s. Apple has been writing Windows software almost since the beginning of Windows.
My prediction: This *WILL* happen, because Microsoft wants to sell Windows to Apple users. As for timeframe? I'd wild-ass-guess predict sometime this Summer Microsoft/Apple will come out with an "official prerelease version" (Microsoft will say it's all them, Apple won't claim any part of it yet,) with tons of caveats and warnings, and terrible performance. Then in fall, Apple will publicly give it their blessing and release Boot Camp drivers, and Microsoft will release the commercial/retail version.