I would think it would be the opponent’s responsibility to point it out, actually. Much like both players are supposed to look for all possible convoy damage on each collect income phase. But if you’re playing with a buddy and you both like to play that way, of course that would be fine (you forgot to land them - they crash).
All aircraft are supposed to be landed safely if possible by the end of non-combat. So not landing them, I would say is a rules violation. Just as with any other rules violation, the opponent should point it out and it can be rectified, and then your question becomes irrelevant.
If someone moves a bomber out and forgets to noncom it, do you think that should crash too? I’m sure the vast majority of players would agree that the non-combat phase is not really over until all air is legally landed (or crashed if a carrier couldn’t possibly pick them up)