What is to gain? It's not like the Windows kernel is bad or useless. You'd have to redo the Windows API on top of a kernel that works very differently. Any gains here are marginal and there's a lot of cost involved.
The NT kernel is pretty darn good, so long as it's not polluted with things that don't belong in it. It shares a lot of heritage with the VAX kernel. A lot of the same people worked on both. If you don't molest it, the NT kernel can (and does) run ultra high reliability things like RAID arrays.
What does the Linux kernel offer them to merit the pain of doing that?
Also, why the Linux kernel in specific instead of adopting any of the other kernels intended to provide a UNIX environment such as XNU, FreeBSD's kernel, Illumos' kernel, etcetera?
They have the technology to switch in the form of drawbridge. Whether they have sufficient incentive to do that or that one option is more desireable than another is another story.
Linux doesn't have the massive data collection apparatus that Windows 10 does. If they released an OS with the Linux kernel + NSA, there would be no reason to use it over some other Linux distro.
Microsoft makes a lot of money from selling and analyzing your data. The main purpose of Windows at this point is the data collection tools.
It's just a matter of time until there's a massive data breach/leak and all of Microsoft customer data is on public display.
https://www.gnu.org/home.en.html#More-GNU
I often wonder why Microsoft doesn't just bite the bullet and adopt Linux as the kernel for Windows.