User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-203.215.121.122-20150121234710/@comment-9595671-20150122121544

Exactly, hence why an arc 400 years in the past wouldn't work. Such info would be revealed near the series end during the present, so going to the past wouldn't really work by that point.