Airlines are largely publically held corporations, as such it is their fiduciary duty to maximize shareholder value. Their frequent flier programs will change to allign with this goal. They clearly have no legal obligation to keep any aspect of the program the way it is now, as spelled out in the terms and conditions. No one is holding a gun to your head and making you stay with a particular airline and frequent flier program, either...

The value of miles could easily plummet, imagine for example if your airline went the way of Delta, turning their miles into "placebo miles" that are nearly impossible to redeem. Nice big numbers in your account, but good luck trying to use them! Mwa ha ha. It could happen.