I would note that the US airlines today are reportedly receiving billions in dollars in aid from the federal government. They already had extremely high levels of liquidity due to the lessons learned during 9/11. Nobody who has frequent flyer miles in a major US airline program needs to worry about these miles becoming "useless" -- unless you're naturally worried about the end of the world and are a prepper.
The situation is different if you have frequent flyer balances or tickets will smaller and less successful airlines. Like I don't believe it likely that Norwegian will be flying across the Atlantic anytime soon (and if they do resume service, it will be quite limited). I'm a little worried about Frontier, simply because their business model seemed weak before this crisis. But I'd be only a little worried. But the major US airlines are in pretty good shape right here, especially as far as frequent flyers are concerned.