That sounds like an interesting new idea for a purge, but a bad idea for a game mechanic to controll thralls.

Most people like the fact that their thrall numbers are under their own control, they can choose with thralls to delete if they want new ones. Very few people would like having the game “control” their thralls by killing the ones they like and care about.

But seriously though, think more on how you could turn that into a purge idea. That would be cool for everyone, rather than just a few people, having more variety in purges could be beneficial to everyone.