Re: Left hand, please meet right hand...
>>Microsoft's operating system engineers had to make do with what they had someone else on another team in another nearby room has written in isolation.
Yup. That's what happened.
The late DEC guys definitely did know how to write an OS - VMS/OpenVMS stand as shining examples of how to do it 99% right.
The DoS/Win 3.11 guys already in Microshaft were, as mentioned, just writing a GUI shell to go on top of a toy(by comparison) OS. Hence the very different approaches, to very nearly everything, that NT had compared to WinX.
The WinNT team definitely didn't like what the WinX team thought was good code and the WinX team refused to listen to any of the good sense the WinNT team talked about how to do things right.
What we have today is an unholy merge of the two approaches, somone having banged heads together until the two product teams gave in a bit and 'worked' with each other to come up with Win 7(ish - this may be wrong; I blame deficient grey cells due to too much beer)