User blog comment:FairyRaven/Fairy Tail Chapter 366: 1000 Souls, Review and Commentary/@comment-6904925-20140110110507/@comment-7418318-20140111091058

Rather than Acnologia, the cause might be Face. That's what it was build for after all, to prevent mages from using magic on a continental scale.

If someone can reverse engineer Face or somehow get their hands/claws/whatever on the blueprints and manage to build others, they could eradicate magic from all of Earthland, except for the curses the Demons wield.