This is from some ideas I was spitballing at on discord with puffin, eric and oval.
The AI generates reinforcements from Manufacturing Centers, which are destroyable buildings on the map, instead of having the reinforcements magically appear at Guard Posts. A reinforcing unit gets made at the Manufacturing Center and it then flies through normal space to its guard post. If there's no path that goes through AI-owned planets then it immediately becomes Threat.
If you blow up a Manufacturing Center, it will decrease the number of AI reinforcements. If you kill all of them, the AI stops getting Reinforcements.
The AI will periodically spawn new Manufacturing Centers; the spawn rate is affected by the AIP, so if you let the AIP get too high and can't kill the manufacturing centers, the AI will actually be getting a lot more reinforcements than it would in the normal game.
If you blow up a Manufacturing Center, something nasty happens; maybe simultaneous Exos and Wormhole Invasions or something?
Perhaps killing too many Manufacturing Centers causes some new force appear and attack the galaxy? Or maybe the AI just starts putting the reinforcement budget it can't spend into Wormhole Invasions.