in the lab where I work, over a thrid of our machines are NT. TO upgrade to another operating system would mean having to buy new instruments that the computers run. THat would kost $80-100K per machine - just not feasible.
For some limited applications you are right. I know of a business the has a custom built aplication that is vital to there work. It dates back to 1985 and has to run on just the favor of unix. The hardware it runs on is 15 years old.
On the other hand would it make any sense to upgrade the computers connected to the instruments? If they originally came with Windows NT, they're probably around 200-400 Mhz. You could probably redeploy newer computers that aren't worth upgrading to handle Windows Vista, but would run Windows NT 10 times faster or more. Windows NT 4 is compatible with Windows Server 2003, so it can be integrated with networks.