I think the ualsurvey provides good insight on what drives customer satisfaction, but I'd guess that deciding where to deploy aircraft would be primarily based on demand and yields.

That said, UA eliminating 30 737s might not lead to any mainline station closures at all. In fact, even has capacity has dropped in recent years there have been more Express to Mainline station conversions (FSD, JAX, RDU, EUG) than Mainline to Express (can't think of any). This is because UA has been replacing mainline with UX on selected flights in markets where they want to reduce capacity, yet maintain frequency.