Interesting question
First off I am with RacerX on this, in addition to openstep and nextstep:
Amiga died in the early 90s (1992?) People still use AmigaOS 3.1. "recently" Amiga inc was formed and amigaOS 3.5 and 3.9 came out, although this still runs on hardware that was made in the late 80s
People still use Atari STs with Calamus software for DTP, atari exited the computer business in the late 80s as far as I know.
I own and use an Apple Newton 2100 with OS 2.1, it still kicks arse
BeOS died a while back, people hacked the Personal Edition (freely available) to create distributions that run on modern hardware so people still use BeOS. YellowTab was also created from the DANO base (now owned by palm).
MacOS 9 dead? I still have it loaded on several machines at work since my dept doesn't have the dough for licenses, and it is not mission critical to have OS X on those machines. as a matter of fact I had loaded OS X on one of them but scanners would not work, so I removed it and used OS 9 where there are drivers for the old scanners that we use.
Given the nature of the MacOS now it can last for a long long time. If at some point both my hardware and software gave way and I could not get another macos x machine up and running I would probably custom build a linux machine (or a BSD machine) and have it setup in a multiboot environment with other OSes (YellowTab, Syllable, maybe longhorn too ..if it is out by them HAHAHAHA
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Personally I do not want to give up my G3 since I have played with the firmware of the DVD to allow it to play other region DVDs, it all else gives way I use it as my DVD player