In regards to the black-masked Ascians, the HW SMN Job quests explained that the black masked ones are not just lower ranked, they're also more restricted in how they can cheat death - unlike the the red-masked ex-Convocation Overlords, the black masks cannot flee to the edge of the Aetherial Sea upon the destruction of their host, and more importantly, cannot possess a living host at all, they can only possess a corpse (just like a Voidsent). So it's pretty easy to see how this can turn out - kill a black mask's physical host where there is no easy access for them to possess another corpse and... ooops... bye bye Mr Ascian (so no fighting them in say a recent battlefield or a cemetery...).

This is how the level 60 SMN Job quest is resolved by setting up a situation where the black masked Ascian is trapped by not having a convienent dead body to flee to - destroy it's host and the Ascian's soul is also destroyed.

Hence this is why I don't think we have anything to fear from any of the remaining lower grunt Ascians as they're easily destroyed. And as there is now not just a cure for tempering but an effective means of protecting against it (to say nothing of now knowing how to summon without causing that annoying tempering issue), I don't see this being an issue for the world going forward.