It's a minimum fleet but they could cut utilization by several hours a day, therefore layoffs/furloughs would happen. Wait for it.
The East has a minimum fleet contained in the Transition agreement (as does the West). Not sure about the West, but the East also has a minimum fleet outlined in LOA 93 that also defines aircraft utilization per day.
I guess the question is... If you can't return an aircraft and still have to pay for it, it is cheaper to let it sit idle or fly a revenue leg?