Not sure what this has to do with UA. It's the same issue for every other carrier as well as Amtrak, Greyhound and for that matter, Avis.

Not remotely worth speculating. Many of the factors go well beyond whether people are willing to take the risk and go to enhanced travel limitations, e.g. pre-flight testing and quarantine. Example: prior to October 15, when Hawaii opened to tourism with strict pre-arrival testing to waive out of quarantine, travel to Hawaii was close to zero and there was almost no service. With the new scheme, travel is inching back. If the restrictions were put back in place, people would generally stop flying to Hawaii again, projected demand would drop again, and frequencies would be cancelled.

While this thread asks about domestic travel, good examples exist with respect to travel to other countries where rules have tightened and travel has dropped.