They make a TON of money selling miles to resellers, like credit card companies, etc. Also, the flying public, Ma and Pa Kettle included, are addicted to miles - I can assure you that if a major airline stopped its FFP cold, not only would they lose a tremendous amount of business, but they'd likely be sued like crazy too.

US is going to be hit with Cockroach v.2.0 due to the loss of bonus miles - assuming their FF masses don't just give them the finger this time and leave outright.

In any case, the loss of revenue from mileage sales would be a no-go for airlines. DL avoided a potential liquidation by selling a ton of miles to Amex.