I think more likely UA would just tell a regional that they are getting the absolute bare minimum block hours allowable per the contract. Because that situation would not be tenable, the regional may choose to close.

Let's say, for example, UA tells GJ that they will only fly 15% of their 2019 block hours indefinitely because that's the minimum in the contract. GJ may not be able to sustain that level of business without any chance for an increase, and may fold as a result. Contract terminated, problem solved for UA.

This would be particularly true for airlines with aircraft on lease wherein the lease must be paid regardless of the aircrafts actual flying activity. The aircraft *must* fly for a chance to break even. That's part of what killed TSA, except it was their own pilot shortage that really limited their capacity to fly, not UA.