I don't think undetectability solves things. Offensive subsystems could survive their creator's demise like two people in a grenade lobbing fight.

Suppose all hid a copy, the merged AI would still be more powerful than any hidden copies, and if it was destroyed everyone would be a small copy again. If there were many AIs, an individual would be banking on its ability to defeat a much larger entity. Offense is more powerful on most scales and technological levels but not by incomprehensible orders of magnitude.