When I am given a seat in F before departure, whether by purchase or by confirmed upgrade, and the flight as a whole has a problem, AA in my experience does the best job of offering alternative flights in F. Delta also makes an effort.

All of the airlines that sell domestic F want that cabin to mint coin. And they have done some serious math on the permutations. When IRROPs happen, F passengers who don't need to travel right away will usually wait a day or two for the best outcome. Those that need to fly immediately should be offered the best alternatives. Passengers should also verify that the upgrade status is DES-F even if you accept an economy seat even for only one segment. The Admirals Club AAngels are quite helpful in times like these.