All the stuff they have recently released as open-source speaks otherwise. The problem why they will most likely never be able to open-source Windows is rather that they bought a lot of third-party components for the system that they simply aren't allowed to open-source. They'd probably have to rebuild half of the system to be able to open-source it, and that's the unlikely part.
I'm pretty sure there's even pieces of the broader system that they don't have source to outright; pinball was an example, and that one piece of Office... equation editor? Anyways, right; even if they absolutely wanted to open source everything right now, they certainly don't have ownership of everything needed to do that, and while they probably have the actual code for most things it's not necessarily guaranteed.
It wouldn't need to be open source. They would make closed-source distro with custom kernel modules and packages.
Just like MacOS.
Darwin is BSD-licensed (it's a FreeBSD derivative after all), that's a huge difference to the copyleft GPL license of the Linux kernel. See e.g. the VMware vs Linux story for the implications of that.