I don't think it would be completely broken, per se, but as a spectacle I think it would be ruined...Obviously not a big concern for 7-a-side, 5-a-side, etc.
Taking things to the extreme...because you'd effectively have a group of players in one box, a group of players in another box and not a whole lot in between. You'd remove the need for midfielders almost entirely and The game would become much more about who can hoof the ball furthest downfield and much less about strategy, which is what makes football beautiful. I guess the tldr version is the game would be simplified so much as to remove any nuance, thereby removing most of what makes it beautiful to watch.
