> I think what Microsoft gets out of it is a lot of people starting to use Windows/ARM

Microsoft and Apple are probably co-developing their x86 to ARM compatibility layer. Why not? Both improve it at the same time, both benefit from more comparability and both benefit from ARM being a serious competitor as you said. Microsoft and Apple can be very pragmatic.

I did notice that Apple emphasised several times that this was "a transitional technology", which I read as only for a few years. So maybe not.