2000 was fine. A nice upgrade for NT 4, but Unix has been running on PCs for a long time, and writing a device driver, while not trivial, was a very well documented process. The fact that, besides PCs it ran on high-performance exotic hardware made scaling up the easy choice before scaling out came into fashion.

As it not being written by Microsoft, that's one thing that makes it the last good operating system Microsoft has ever sold - someone who knew what they were doing wrote it for them ;-)