Fenixius said:
If you extrapolated TF2 onto a 64p map, it'd be ridiculous. The game wouldn't hold up. I've never seen any game designed with the intention of having such a high proximal concurrency, and I don't really see one working at all. Either it spreads out too thin, and other players have less immediate impact, or we go the other way, and everyone affects everyone, and noone plays because it doesn't make a difference.
That's what I'm getting at - it's not viable in a conventional videogame. Something like 1 vs 100 might work, but I think that's a special case outside the norm for established videogames.
You might be right, for the average player. Having participated in IC-ArmA, for ye olde Armed Assault, the 100+ player battles WERE terribly laggy, crashed often, and were (due to the game, hardware and ping issues) often frustrating. But we continued anyway, because the experience was just so awesome. I remember one of my first games. My assignment was sitting inside (alone, I might add) a Stryker IFV, at night, watching a town making sure no enemies were going to hit it or pass through it. Gameplay-wise, I was the equivalent of a canary, since I wasn't actually expected to fight back any enemies, just get killed and thus warn the rest that there were enemies around.
I didn't shoot a single bullet or die a single time, yet I had a goddamned blast just listening to the comms, watching the flares, the explosions, the odd helicopter flying by. Oh, and staring into the greenish, NV-goggled forest looking for anything that might suggest an enemy sapper.
Later on I got to experience all the joys of being a tanker, including sitting by myself on top of a hill fighting back hordes of enemies, letting our infantry advance just a bit up the flank - or taking out the whole enemy tank division in one quick action, opening up a window for a push. And sometimes, I'd just sit somewhere staring at nothing and BOOM I'd be dead and waiting for my damn tin-can of death to respawn for ages and ages, or a helicopter would come from nowhere and blow me to hell, or I'd be out-sniped by that other tank, or I'd be stuck in the middle of nowhere with my legs busted and no help in sight. Sometimes a whole game could be like that. And sometimes, sometimes a game would be pure gold.
The only reason, really, why these battles were sometimes less than phenomenal, was the lack of players! A higher concurrency in this game (or, rather, a netcode that would allow for it) would be absolutely perfect. So there's a game designed for it, if there ever was one.
But I guess that I'm speaking here as an OFP/ArmA/Arma 2 player, which, as the Internet generally agrees, is a species of gamer all of their own

Because you are of course entirely correct in that TF2, public-server, 64 player games would NOT ever work reliably.
Edit: So in conclusion, the OFP/ArmA/Arma 2 series of games IS designed for a very large player concurrency, even though the engine and netcode are NOT optimized enough for this to be technically feasible. The dream-come-true of this game would be a combined-arms assault where infantry, supported by artillery, IFVs, tanks and aircraft would make a push against a fortified position with all the same assets as the attacker - and every cog (numbering any amount, really) in this wheel would be controlled by a player. It works, because it's based on real-life military logic, which is of course one of the larger and more visible places in which "players working on the same overall team, in the same location, pursuing the same or at least related goals" can be seen.
I am aware this is a pipedream, and therefore not really 'worth' pursuing, but it's a matter of technical drawbacks, not game design or some kind of idea of "impossibility", that makes it so.