Strictly speaking, I don't think there's any technological reason why you couldn't replace the shell, although practically I'd bet the boot chain will refuse to

Strictly speaking, I don't think there's any technological reason why you couldn't replace the shell, although practically I'd bet the boot chain will refuse to load e.g. a replacement compositor that's not signed by Microsoft, not to mention the lack of API documentation at that level. I mention it since sometimes I daydream about trying to force a newer Windows runtime/kernel to load an older shell... Still, in a manner of speaking, this is encouraged for the Server SKUs, which IIRC Microsoft is trying to push the "GUI-less" version which you do all configuration on remotely with powershell and DCOM and the other half dozen RPC interfaces in Windows. It's not a replacement shell, but it is an example of running WinNT with the UI front-end hacked off.