I would say there is a more likely than Windows becomes open source and (some) Linux distros will switch to use the windows kernel than the scenario you put forward.

The amount of work it would take to switch Windows to use a linux kernel is mindbuggeling. One thing is the actual task of switching the kernel but documentation, tools suppor, drivers, end user training etc etc makes this scenario unrealistic.

It wouldn't even bring any benefits to Windows. It already has a perfectly fine kernel

I think this would be a shame. I’ve always been a Linux guy pretty much since I was old enough to have my own computer, but NT is a fascinating architecture capable of running multiple different system abis (POSIX, Win32) side by side and was designed to allow for doing just that.