So, halfway through the game I learned something a little distressing - if you don't manually activate the alternate victory conditions at the start of the game, the only way to win is through eliminating all other factions. It's especially annoying because I'd already spent five hours on a Huge map, took out all but two of my rivals, and forged alliances with the remainder. For the last hour or so, I've mostly been waiting for the diplomatic victory to trigger. It was only when I looked it up that I learned it was never going to come.
I've decided to count that map as a personal victory, regardless.
Although I wonder how far the game would have allowed me to go. I set the map to be deliberately underpopulated, six factions instead of ten (mostly because I don't like repeat factions and there are only six in the game) and managed to destroy my two closest enemies with decapitation strikes early in the game. The other three factions were in a completely separate star system and by the time I started signing treaties, two of them ganged up to eliminate the third. Now, there are three more-or-less empty star systems with dozens of planets between them and plenty of room for all three of us to expand. It would have taken hours for us to even get to the point where our expansion would come into conflict.
Could we have existed in that equilibrium forever? Is there some sort of mechanic that would have forced my hand, sooner or later? I'm almost curious enough to keep playing the same save file, despite the likely pointlessness of such a course.
I'll save that for later, though. Just in case my game with fully-enabled victory conditions doesn't work out.