My take?
Big three in deep kimchee. Too many fleet types to be nimble enough to respond to a changing marketplace and overhead will eat them alive.
Worst case? F9 and NK merge. In that event the two survivors of the Big Three are in very deep kimchee.
SWA I think will hold its own. Probably Alaska too
I realize that others opinions will differ but I think that international travel will be the last to come back and 20-25% of business travel never will return.