Quote Originally Posted by shinta|hikari View Post
They don't have to perform D-mails. They just have to undo the previous D-mails' effects. It just so happens that cracking SERN achieves that, or so they think.
That's true, but they can't undo that effect without changing the past. And the only real way they can do that now is to use another D-mail.

Cracking into SERN to delete the records of *whatever it was that I forgot, lol - their data?) doesn't mean none of that ever happened. Anybody who's been reading that up until now and has acted on it up until now will just notice everything's gone. Using that rationality, I just can't see any way of changing world lines other than doing something to the past - ie, send a D-mail.