That is indeed an interesting what-if.
My 9600 came to me with BeOS installed, and I dumped it for OS 9 almost immediately. I'm wishing I'd saved the install. One really interesting thing about BeOS is that-as far as I know-it's the only OS that can fully utilize both processors in my 9600. Even OS X Tiger(which @LightBulbFun was able to get running on 60x series processors) only sees one of the processors in the system. OS 9 knows they're both there, but can't do anything with them.
NeXT had a good case if nothing else due to the Steve Jobs connection.
Things were a lot more interesting when we had a bunch of completely different OSs running around. Now everything is essentially either WinNT derived or a *nix variant...I know there are a billion and a half Linux distros out there, but under the hood they're more alike than different. OS X does sort of stand out because it's maintained by a major computer company and has what is(IMO) the best GUI around, but is still BSD Unix underneath it all.
And, not to ramble too much, but NT 3.5 and NT 4.0 were compiled compiled for a handful of architectures. The NT 4.0 disk I just looked at lists PowerPC, MIPS, and Alpha in addition to a couple of x86 processors. The only problem was that in the mid-90s, the most common PPC machines were Macs and the most common MIPS machines were Silicon Graphics. WinNT will run on neither of these platforms-AFAIK its PPC support is limited to the handful of PPC machines IBM made, and I don't know offhand who other than SGI used MIPS processors.
Do you know why OS X doesn't fully utilize both processors in the 9600? I thought that symmetric multiprocessing was one of the features that Apple always promoted back when they announced OS X and prepared to make the transition to it.
I'm too young to remember when there were a bunch of completely different OS's on the market, but I do agree that it was a far more interesting time than what we have now. Everything is so standardized these days that it sometimes gets boring, and I doubt that will ever change. I highly doubt that anyone will create a completely new platform that's not a descendent of something that already exists.