Bluegill will prey on them, to be sure. The problem is that the threadfin take pressure from the bass off the bluegill to the extent that the bluegill overpopulate, and when that happens their growth is severely curtailed. If the bluegill don't overpopulate, there's still the issue that all of those small threadfin are directly competing with the bluegill for food i.e. plankton.
But that's just my experience. Eric has fished lakes in FL with threadfin that also had large CNBG, so perhaps TSH are a different animal there than they are in TN. Florida LMB are not northern LMB, and that may have a lot to do with the equation.
