Nobody is talking about Terminal C or even A-2. They are talking about CUTE gate access in the new terminal 1. If CUTE gates are not being used and the preferred carriers are not using them, other airlines should be allowed to use that. And if they don't meet their usage requirements, they should no longer have preferred access to them. I just don't see how AA could continue to use up 7 gates if it becomes CUTE. And if UA operation can mostly fit in Terminal C, they should not be holding half of the gates at the new terminal 1 and not allowing other carriers to use them.
If we are talking theoretically, then yes, I agree that CUTE gates *should* have a use requirement, and an airline should not be permitted to squat on gates. But, at T1, only the gates themselves will be CUTE, and there will still be airline-specific leaseholds (for which United's space is substantial), and gates allocated to air carriers on a preferential-use basis. The use requirement on the T1 gates is going to be lower than most would expect (something like average of 3 daily turns).
My discussion on this topic is aimed at the idea the Port Authority will be allocating T1 gates principally on the basis of capacity ratios in the post-COVID environment, especially Summer 2020, which is nonsensical. This is an aberration, and while it will take time to get back to 100%, we aren't going to see United suddenly pushed out of T1 because of a temporary reduction in flying necessitated by the pandemic. No chance, and, as I mentioned, most of those gate allocation issues have already been agreed upon, at least in principle.
The other notion, generally unsupported by evidence, is that United squats on gates at EWR, exclusive or not. During normal times, United is usually between 5 and 6 departures per gate, per day, some gates more than others, reflective of the regional/mainline narrow/mainline wide/international arrival mix. That means its usage rate is generally better than DL at JFK (more comparable operation) but not as much as DL at LGA (higher-frequency, smaller aircraft), and several-fold better than AA at JFK. It's roughly equivalent to B6 at JFK T5.
Terminal 1 is not exclusive use to UA. They are CUTE gates. AA & DL terminal at JFK also have plenty of other carriers in there. DL actually has to battle other carriers for space at T4. As for other hubs around the country, the non-hub carriers can add as many flights as they want in the CUTE gates. I doubt they will be forced to stop flying them when the hub carrier resumes activity.
AA allocates space to other carriers in T8 at its discretion, as it is under an exclusive-use arrangement for that terminal, and while T4 is technically CUTE, virtually all DL gates at T4 are at least preferential-use, and some exclusive. CUTE gates are not the free-for-all you seem to think they are, though. If the space is already assigned to an airline, it is incumbent on the airline to maintain a service level for that gate/time to remain preferential or exclusive. Generally, in the case of the NY airports, these are reviewed at long intervals (not month-to-month) and are often part of the horse-trading between airlines and the Port (e.g., part of a negotiation for tenant projects, improvements, capital investments made by an airline, etc.). In other words, it's not considered in a vacuum.