Big studios didn't convert to NT, they converted to Linux. SGI ended their own reign by not being able to compete with Nvidia and AMD/Intel. Even now all those studios use csh since that was the default shell that came with Irix.
SGI made a few NT computers as an attempt to sell PCs which of course didn't work out due to not offering much except for a name an inflated price. Linux was coming around at the time, but SGI wouldn't have been an early adopter of Linux by any means since Irix was one of the jewels of the company, and helping adoption of Linux would have helped people transition to commodity PCs.
NT was a part of the transition for much smaller groups or individuals since they could buy a dual processor pentium II/pro and put a decent amount of RAM in them along with an Oxygen card that would accelerate OpenGL.
Large studios had a huge amount of infrastructure built up in pipeline scripts and workflow, proprietary software that might have used POSIX APIs and GUI libraries that weren't on windows, and big network file systems that tied things together.
I was working in the industry at the time, and my feeling was that while a few big studios went with Linux the vast majority of studios (especially among the smaller studios) went with NT.
Every big studio and the majority of medium studios converted to Linux once Houdini and Maya were ported. At the time the industry was dominated by five big American studios and 3-5 big London studios, all of which went the Linux route, not the NT route. Maybe you worked on multiple small studios that jumped on windows, but that wasn't the norm nor was windows the driving force is providing a smooth path from Irix.
Big studios didn't convert to NT, they converted to Linux. SGI ended their own reign by not being able to compete with Nvidia and AMD/Intel. Even now all those studios use csh since that was the default shell that came with Irix.