NT 3.5.1 was excellent OS. NT 4 was also an excellent OS, but they did move the graphics system into ring 0.

This did a few good things, and IMHO, some bad ones.

The good was a foundation for a much more efficient graphics experience that was easier to achieve. Many believe the 3.5.1 way, with graphics in Ring 1 was just as capable, but just a bit more difficult to make shine.

PC gaming really wanted the ease of XP with the better kernel in NT.

The bad thing was the inability to theme and customize the window manager, desktop and other features like we could do with the X window system at the time.

My NT 3.5.1 system was customized and it rocked! Wish I had some screenshots...

NT 4 was basically the win 98 GUI and ... meh. It worked well, but was no fun.

Accelerated 3D cards were becoming a thing. NVidia along with others made great cards that could run games, and nVidia supported full OpenGL, which had the higher precision found in the UNIX world, and on Sgi IRIX hardware in particular.

People could pay $5 to $10k for an NT PC and do things that cost $20k and up on SGI, and do them often more quickly.

But it would crash, and being in ring 0 meant crashing the whole box hard. Was kind of an ugly time.

And money talks! People put up with the crap, Intel produced multi processor and then multi core machines that NT was able to handle.

End of one era (IRIX, Solaris, others) and the beginning if another one, (Linux, NT, eventually MacOS)