I can understand where OP is coming from. If one has lifetime status due to flying a number of years ago but hasn't been flying DL much (or at all) in recent years because they are on a different carrier's program, it seems to make sense to me for DL to try to win that traveler back. Alas, DL disagrees (which is of course their right) and I don't think there is anything the OP can do about it.