Well, you killed Greenscale and Alsbeth fairly quickly in original Rift. Like less than a 1 month apart IRL time. Both Alsbeth and Greenscale were major players in the Defiant and Guardian early storylines. Respectively.
Then we killed Akylois. Then Maelforge and Laethys.
So yea, seems the Ascended don't fuck around much.
Also, it has been explained that the true form/nature of the "dragons" [psst, they're not really dragons] lays beyond the veil. In quasi elemental planes. So you can't really "kill" Regulos on Telera. He exists as an entity in some ether of the universe.
Long ago, Scott Hartsman dropped a tiny hint that at some point the Ascended would travel through the planes via Rifts. Everquest 1 style. Trion has been tight lipped about that ever since though.