You are assuming that one of these carriers will go completely under, and no other carrier will buy them in the process (or take over the miles). In almost every single case of an airline demise, some other carrier took over the routes and the mileage responsibility (just look at Pan Am, Eastern, TWA, etc.). Sure, there are exceptions (such as Midway) but these are fairly rare. I suspect that if either USAirways or America West were to go under, some other carrier (though there isn't much spare cash out there!) would buy them out. It is a cheap way to buy planes, routes and loyal customers.