> What did Windows, OS X, and Linux bring to the party apart from hardware improvements just to be able to run them in the first place?
As a '90s gamer, DOS was my main environment, and Windows was largely ignorable. RISC OS just wasn't available for my x86 machine. I'd used RISC OS at school just as I had used Atari ST and Amiga equivilents at friends' houses (and was jealous of their sprite based games libraries).
OSX seems to have served Apple and their users well, especially those (mainly DTP and music professionals) who hung on in there during the '90s. I've not heard of RISC OS being considered by Apple before they brought in NeXT.