If we are talking about possible future scenaros post Covid19, I would see only XJT and C5 as the 50 seat operators for United.

Chicago and Denver would go to XJT and EWR/IAD to C5. XJT had it's focus shifted West with the demise of TSA and I can't see that changing. I also can't see anything bigger than a 50 seater for most of the flying outa EWR and IAD.

The fact that United have ownership stakes in both these operators also plays a major part in this theory.