I don't think we need to worry...unless anybody can give a specific example in the past 15 years of a legacy carrier in the U.S. going under without complete salvage of it's frequent flyer program. Correct me if I'm wrong on that one (I'm sure somebody will). In anycase, I'm still rooting for US.
I just happened to book a trip this weekend with all of my US miles because the wife and I wanted to get away, the tickets were available, and the price was right
My wife still has enough for a free domestic ticket that we will hang-on to for now.
You land a million planes safely, then you have one little mid-air and you never hear the end of it - Pushing Tin