Absolutely what Chemical Art said. No Minor Faction should be necessary to beat a game of AI War in a reasonably plausible fashion. You should not, under any circumstance, need anything like Fallen Spire or Showdown Devices to finish the game unless you've made terrible mistakes and raised AIP to really high levels, kept core worlds on alert, etc, in which case... the AI should just kill you rather than there even being a stalemate in the first place. I mostly play without Minor Factions, myself.
In this sort of case,
eventually the AI will take aggressive action towards you with the sheer amount of "overflow" that will start being sent at you via border aggression, and if you let it keep going for really long, scrap waves. Once all the worlds fill up, which they will
eventually, that can start being a painful amount of active aggression.
However, depending on your AIP, the
rate of this aggression may not be enough to take you out. Although +AIP over time would solve that, not everyone plays with +AIP over time.
But then again, if you asked for an opponent whose max-strength does not grow over time if you don't do anything, stalemate prone situations if you don't take action as well is sort of a natural result.
That said, back during the AI HW buff period, Keith has stated he would like to see someone test out the new AI homeworlds without resorting to cloaked warhead cheese or superweapons, and see if it still seemed reasonable. Sadly, I don't think anyone got back to him on that, and maybe that's why it seems that AI homeworlds got overbuffed; no one vetted them using non-superweapon, non-cheese tactics.