well it maybe 10,000 dragons - but there are many other Dragon Slayers out there whos magic is what it is...to slay dragons, we also don't know the strength of any of them. 9,950 could be regular pawns who suck ass. We'll just see though.
-- But if future lucy is trying to undo that and prevent it from happening then; most likely Mashima is find a way to end the apocalypse during the first hours or so of it starting, so that we see it actually happen before everyone is killed cause someone is gonna try to undo it.