The other part is significantly better hardware support (including powersaving and wider range of devices) which would also be completely undone by a unixy kernel.

I really kinda fail to see what would be gained by rewriting a piece of kernel that works just fine (well, except that ./ users would finally declare that they were right all along!)

I think the hardware support point is moot because if Windows did switch to Linux (Microsoft wouldn't but we are playing the "what if" game here) then hardware manufacturers would obviously port their drivers to Linux else nobody would buy their hardware. We've already seen this to be the case on Android for example. Plus Linux already has reasonably good support for older hardware.

I should stress again, just in case anyone misinterprets my comment, that I don't for one second think Microsoft will not should switch to Linux

They would port their new hardware drivers, maybe. It's the 20 years of backwards compatibility that keeps Windows strong in many places, especially government, military, corporate.

I did also address that point in my post you're replying to

Unfortunately they aren't running together. They run entirely separated in isolated subsystems with no way to communicate between the two. The GUI makes them seem more integrated than they actually are.

Linux w/ a windows vm makes more sense for that use case though

Not always. If the windows only software needs access to a hardware device (e.g. GPU) then there's still a significant barrier to setting that up. Hardware support is kind if there but some vendors dont allow it with consumer cards and you'll need two of them if you want both desktops at once. Along with that it's not uncommon for even new hardware to get details about doing this wrong. Currently amds threadripper and corresponding motherboards don't work with it due to a bug with power management.