That BeOS didn't become a serious, long-term contender in the desktop OS market is a sure sign we're not living in the best of all possible worlds.
That Amiga never achieved any success outside of niche SFX markets is a sure sign we're not living in the best of all possible worlds. (Although that's more 80s than 90s.)
There's a timeline out there where WindowMaker/GNUStep became the dominant desktop for Linux too.
In many ways, I think the state of modern personal computing is evidence we are, in fact, in the darkest possible timeline for it.
If you have not seen it, check out the BeOS-inspired Haiku