You’re describing the importance of coherence. You can only change so much before you are no longer the same thing.

Ship of Theseus, and all that, yeah. That's part of it, but I was more talking individual subversives. You don't have to replace every part of the ship/nation, to change it drastically. You can leave most of the parts the same, but corrupt them. Think of it like termites versus replacing the parts. Termites, you'll have the same ship and pieces, but it will have been eaten away from the inside.