I think the demons are still underestimating just how immortal the Idaten are. Idk if it's every been explicitly stated, but I thought it was implied that they are truly immortal since they are made up of ideas. Even if they were completely erased, they'd probably just start to regenerate from nothing, just like how they are created. I might look back through the early training to see if any of them have had their heads crushed or something like that.