For reference, older Linux desktop options were... well, I'll just say they played a lot like Dwarf Fortress.
Otherwise I only saw Solaris or commercial types of unix (like what came with IBM servers) in use in companies I directly worked with. I understand that linux was already rising pretty strong, but tried and true Unix/BSD systems were definitely seen as the more 'serious'/professional choices.
Which was interesting because then Linux became faster, and had things like better SMP support that didn't get improved in BSD to give it the performance edge until 2007 or so...
(Disclaimer: I am one of those programmers who only switched to Apple because of the tiBook, which was a damned sexy Unix workstation in spite of the fact it was also a damned sexy laptop.. and I used it for developing apps for Linux and SGI Irix without much hassle.)
You had to take care to use libraries/APIs that were available at all three systems (no dlopen() or dlsym() at the time in OSX!), and take quirks of the OSX linker into account.
(Disclaimer: I was getting angry at Apple at the time. In a few years, they went through OS9->OSX migration, getting rid of Classic mode in emulation and now switching to Intel. They were breaking compatibility again; it meant buying new software like MS Office again - looking at the PC side, they didn't have such problem. So PB12 was my last Mac for a long time... until 2015, actually).
To me it's when the Intel macs came along (10.4.4 I think) that new adopters really started picking up steam, but the ecosystem they helped build wasn't really there yet.
At least, that's my perception.
Not to say that was the entire market, but it was a good chunk of it.