The NT kernel in and of itself is very small, simple, and remarkably well designed. The Win32 subsystem is what is horrid, and that's 99.9% in userspace. There's no reason that they can't create a whole new subsystem along side Win32 where they improve everything, but I don't see that happening.
Is there current documentation on that? I remember NT3 had a very microkernel-ish design, but I have also read that a lot of its elegance was compromised since NT4.
NT has had some growing pains architecturally speaking, but it's been handled remarkably well. Probably the best thing to ever come out of MS, especially when you contrast it to the mess that is the Win32 subsystem.
There's absolutely no reason for Windows to switch kernels.
As for virtualization support, you're wrong on two counts: 1) with the creation of Hyper-V, a number of facilities were added to NT to support something akin to paravirtualization, and 2) MinWin has absolutely nothing here that NT doesn't have, being a subset.
1. Some of us have the preposterous notion that the NT kernel is superior and more modern than the BSD kernel.
2. The NT kernel architecture is not new by any means. It's mature and robust.
3. Whats wrong with their windowing system? The DWM is pretty speedy, efficient, and stable.
4. "This would also open Windows for serious software development"?! It's currently the number one consumer (some, including me, might argue iOS actually is), and the number one business and server OS. How much farther ahead of the pack does it need to be before you'd consider it "serious"?