I've long thought that Windows NT might make a pretty good OS if it weren't married to the Microsoft Windows interface. I know I'm abusing the terminology here, but only because the concept of a replaceable graphical shell is utterly foreign to an OS which began as one of many replaceable graphical shells for MS-DOS.
2. NT did not begin of as a shell for DOS. You’re thinking of the other Windows desktop lineage that ended with Windows ME.
3. Even Windows 3.x and 9x (the OSs that did use DOS as a bootloader) supported alternative shells. It was literally just a one line change in a config file. I authored one such shell for Win95.