That's what I'm suggesting - the catastrophe proceeded to the point that all members of the cluster were either ejected (far away enough to be indiscernable rel

That's what I'm suggesting - the catastrophe proceeded to the point that all members of the cluster were either ejected (far away enough to be indiscernable relative to the quasar) or absorbed by the quasar core. Thinking some more about it, there are probably some ways to constrain the amount of time it takes for a past runaway to have occurred that rules this explanation out.