Gygaxian D&D is competative. It's PCs vs the Dungeon. And sometimes PCs vs each other. It's also constrained into the dungeon most of the time - calling it open ended is dubious. As for very difficult to break, oD&D was playtested by dozens of players, used to the competative nature of wargames and looking for any advantage they can find. It is only hard to break because it got sandblasted in playtesting in a way arguably no RPG has been ever since. This isn't inherent.
I'm not sure I exactly buy all of that. In reverse order:
True, there's nothing inherently un-breakable about RPGs nor cooperative games, in general. Quite the contrary, balance is /more/ important in cooperative games because you're playing against the game, not against eachother, and imbalances can quickly marginalize a player (so he might as well not be playing at all) or remove all challenge from the game. In a multi-player competitive game, if one player uses a loophole or imbalanced choice to gain an advantage, the other players can often 'gang up' on him in whatever way the game allows. In a cooperative game, you're actually playing 'right' if you do whatever you can to help the the player keep his loophole advantage going.
Old-school D&D breaks easily and often, it's just most of the breaking is done by the DM, not the player. There's none of the facile over-rewarding of system mastery or player-driven 'builds,' but every trap, trick, monster & magic item is utterly arbitrary, and any decision could turn out to be a catch-22. Thus the two most familiar DM'ing styles of the classic D&D era: Monty Haul and Killer.
While the early 0D&D campaigns were traditionally dungeons, other environments were being used quite early. Blackmoor, the 2nd 0D&D supplement, had all sorts of stuff for underwater adventures, for instance, and 0D&D has some oddball rules for wilderness adventure.
I will agree that the little I've heard about the earliest days of 0D&D, especially when Gygax & co were developing it, certainly seem to suggest a certain competitive spirit. And, the catch-22 nature of many traps and cursed items we see suggested in AD&D makes me think of DMs trying to deal with the consequences of such competition, and getting the party to have to depend on eachother and maybe not just fight over the latest goodie.
Are you saying that Gary Gygax did not write an RPG?
The original books said 'Wargame' right on the cover.
In order to deny that brokenness is meaningful, you need to throw vast swathes of roleplayers including Gygax, Arneson, and the entire Lake Geneva Tactical Studies Association out of the hobby.
And I stand against throwing people out of the hobby.
The better-balanced the game, the more the expectations, attitudes, and styles of those at the table playing it can vary before they start ruining the game for eachother. A less balanced game tends to meet only certain expectations or over-reward only certain styles, a badly-balanced enough one still likely has one (often quite obvious but also often quite bizarre) 'right' way to play. If you're playing with a sufficiently homogeneous group, you may not 'need' much in the way of balance, and if you find a game whose imbalances meet your expectations and over-reward your style of play, you might be very satisfied with it, indeed...