Great finale all around. I especially liked making it clear that King and the Collector are going to stay friends, which means they'll have each other long after their more mortal friends are gone.
Part of me wonders if the Archivists were originally intended to be the final big bad. They seem like a logical escalation from the Collector.
Side Note (Steven Universe spoilers): I'll always defend how that show ended, but yeah, it's pretty unrealistic. SU Future and Unleash the Light gestured mildly at the idea that some people are beyond saving, but not with anywhere near the force with which Belos is dispatched here.