I'm not sure where you got the idea that AI spies don't have a respawn. They do, just like human spies.
Well considering I killed 3 spies belonging to the same AI every turn for 9 turns in a row in industrial era.
If the current system is heavily weighted towards nothing happening then the current system have events triggering way too often. I was thinking something like every 5-10 turns with a 70% chance of nothing happening on top of that.