The core monk was good at one thing: not getting puppeted while also not laboring under the paladin code of conduct. Barbarians are okay, but are vulnerable in the surprise round and don't have enough decent rage powers other than superstitious to be interesting. Against a GM that aggressively uses charms and compulsions and doesn't cater to paladins the core monk is the martial: doing a little less damage is worth it to not be doing your damage to your wizard friend.
The APG came along and zen archer and the monk became the best archer. The barbarian became worth playing, but still has that surprise round vulnerability that monks lack. Antipaladins provide an alternative to the paladin, but are even less playable at most tables.
UM brought quiggong powers which let you ditch the anti-feature of spell resistance. None of the non-paladin martials gained anything to encroach on his turf.
UC brought style feats, the maneuver master, the tetori, the sohei, and the flowing monk. Now the monk does a bunch of things well. The monk is the best mounted combatant, the best at maneuvers, and can be pretty good at being monkish as well. And they still don't get puppeted while other non-paladin martials still do.
The ACG gave us more style feats and brought out the competition for the future unmonk. And that is a good name for it because it's not going to be filling the monk's niche.
The unmonk doesn't laugh at save or puppets. He doesn't fight like a sohei. He doesn't grapple like a tetori. And most damning of all he doesn't out-fight the brawler. He doesn't really out-save a brawler either. Any stat spread the unmonk can live with the brawler can do better with. The unmonk hits slightly better but the brawler has options and is leagues ahead at combat maneuvers. The unmonk may need wisdom more, but he doesn't need strength, dex, and con less.