It does, and it does not. If United wanted to ensure the lounges were open to cover any flight delays, sure, a provision for same can be negotiated into the agreement. As it stands now, where the OP is asking if it is reasonable to expect lounges to remain open with extended flight delays, this is the reason they probably do not. Not an excuse, just the reality of the situation. When the lounge staff were all UA employees, it only would take a manager-level position to authorize overtime, and the club could stay open as long as they liked. Now, it's not so easy. Plus, don't forget that in the "good old days", with limited exception there was no hot food service and clubs weren't nearly as busy (especially at a hub). Thus, the operation wasn't quite as labor-intensive and it maybe only took a handful of staff at the bar, the front desk and elsewhere in the lounge to keep pax updated, drinks topped up and the hamster-feeders full. Simpler times.

With that said, I think closing the Polaris Lounge at EWR at 10pm is particularly stingy, as any delays at all to the dozen or more Polaris flight departing after 9pm means pax have to be thrown out when the club closes. The United Club at C-3 is the same way, but perhaps even worse as it serves chronically-delayed domestic flights at that hour.