I've been flying ORD-PIT for a few years now. Every summer UA goes to a lighter schedule into and out of PIT. They typically downsize planes from A319/A320's to E170 or smaller! In the past few years after labor day they have shifted the mainline planes back to PIT because yes the planes are usually full.

In my thinking they have just decided that while they can fill the planes to/from PIT, they can squeeze even more revenue by shifting those planes to other seasonal routes.

In other words I would not take the current schedules as a sign that UA is pulling back from PIT. In the past 2 or 3 years they have actually gone the opposite direction (i.e. they began direct SFO and LAX routes when US dropped them).

While I haven't taken a look but I suggest you look at the timetables for late September/October as that might give a better indication of the PIT market.