User blog comment:IamJakuhoRaikoben/Chapter 327: My Part Too, Review./@comment-3075024-20130412211140/@comment-7418318-20130413075847

I know and that's exactly the time I was talking about.

400 Years before the dragons ruled the entire world and then all of a sudden they became extremely rare, so my guess would be that they "disappeared" by actually traveling to the future.

If they are prevented from traveling into the future to live there, that means they'll stay in the past. Unless they decide to live on a remote island or somesuch, that many power beings would surely cause great changes to the timeline we know.