IANYAL, but United has an extremely difficult case to make, and I doubt that they will prevail.
1. Air Wisconsin has done nothing wrong unless there was contractual language present which extended the duty of good care to a prohibition against working with United competitors. This seems unlikely, given that AWAC has done flying for other airlines while offering commuter services to United.
2. There is no unjust enrichment issue present. United received a benefit from the Air Wisconsin contract in the form of continued (and indeed, expanded after ACAI left UAX) flying in exchange for the consideration provided to AWAC in the form of bonuses.
3. United's failure to anticipate a prolonged stay in bankruptcy does not create any sort of obligation on Whisky's part, nor does it raise any issues of unconscionability with the AWAC contract itself. The parties had equal bargaining power and access to pertinent information at the time the contract was formed.
In short, it's United and its creditors posing as the bad guy while AWAC is the good guy who has done nothing wrong.
Live life to the fullest.