No ET! Ever?

[Aoi Magi]

Yes they might, but that doesn't really increase the chances of communication, rather as more civilizations destroy each other, communication possibilities will only go down.

Mutual Assured Destruction is the restraint. Otherwise, yes. Any reduction in the number of civilizations would reduce the chances of communication. Unless I suppose one benevolent civilization became dominant. It could encourage peaceful coexistence under threat of judgement and wrath. Hmmm...sounds familiar.

....till eventually the power corrupts them