I guess there's a difference between "flying" and being recognisable as a US flag. If the flags are bleached white, then it is clear that they must have surrendered to the solar UV flux.
It would be really interesting to visit an Apollo site after nearly 50 years and see how the remaining hardware and other exposed material has fared. Would the Apollo 11 flag fare any worse than the other flags, being in contact with the lunar regolith? One might imagine that a 'flying' flag, especially a bleached one, would have a high albedo and be relatively isolated from the surface temperature fluctuations- reducing degredation somewhat?
And some other possibilities for how the lunar environment could preserve or destroy items- what if I left a pot plant unprotected on the lunar surface? I mean, obviously, the poor thing would die, but what if I came back in a month's time? A year's time? Two years? Ten years? What would a plant exposed on the lunar surface look like after that amount of time?