On the one hand I dont mind if slaanesh remains imprisoned as this Newborn seems like it will serve as an avatar of slaanesh, moreso than any greater daemon could, which is a huge bone to throw for slaanesh still being a major active threat to the realms.
However, I dont know how many times I could stand slaanesh being close to being freed and them doing the same plot over and over again. Archaon was close, the new seekers warband find the gateway to his prison and it is highly likely we might see some sort of hedonite crusade lead by the Newborn in trying to release him, maybe with Archaon although it depends how they want to handle him since Wrath of the Everchosen.
I dont think they will kill of this Newborn character in the Broken Realm series as it gives them a much more tangible way of demonstrating the threat of slaanesh in the realms.