My theory is that the next version of Windows is going to use the Linux kernel. They can keep Windows 10 going for a few years yet (maybe up to 5?). That will give them time to work on cross-platform .NET, so new development will be an easy switch, and good quality Win32 emulation, which will be built someway on the work done on Wine.
What do they have to gain using the Linux kernel? As WSL shows running Linux userland applications on the Windows kernel is perfectly feasible, however, running Windows applications on a Linux kernel (aside from GPL nightmares) is a different story. The two architectures are quite different, but NT was built from the start to support different 'personalities' (cf. the OS/2 and POSIX subsystems).
There is an ancient story about an old ship with all its parts replaced and the question was when does it stop being the old ship and become a new ship? If Windows has a Linux kernel, is it still Windows? I guess Windows is whatever Microsoft calls Windows?
DOS had no kernel - it was just a bunch of code that ran on top of a bunch of PC-compatible hardware. There was no separation between kernel-space and user-space.
You are applying an overly restrictive definition to something that is just the nut analogy, which is a vague analogy that does not imply "spaces".
MS/PC-DOS, and DR-DOS, were not just "a bunch of code" and had a definite structure when it came to operating system design. They comprised the basic disc operating system, the basic input/output system (incorporating built-in and loadable device drivers), the command processor, and the housekeeping utilities.
The kernel of (386 Enhanced Mode) DOS+Windows analogous to the Windows NT kernel being discussed here is the VMM and the VxDs, with krnl386.exe as a distinguished Extended DOS program running in the distinguished system VM.
