I think it is pretty telling that all legacy carriers (AA, DL, UA, US) that reward frequent fliers have been or are in bankruptcy, while all LLCs (B6, SW) that reward high spenders have NEVER been in bankruptcy.
No its not. Those carriers cherry-pick routes, don't use regional affiliates, and have higher average fares paid. (and of course, don't offer premium cabins), and don't serve int'l routes (except B6, and they're not in great shape anymore financially, but they're not that old). Also, you can't use the extant examples of discount airlines, there are plenty that have gone under.