This game has no co-operative mode at all; why couldn't an AI choose to "side with the winner" and also "win'?
To say the game as NO co-operative mode is an exageration, there are DOF and shared DOW and trading for mutual benefit.
But yes more co-operative diplomacy is needed and AI should be able to "side with winner" as you say.
What I'd like to see is that as the game progresses and one or more players approach victory the world should split into factions depending on who they would rather win (cold war style).
They would then hinder the other factions through a combination of CS stealing/culling, trade embargos, diversions (troop build ups on boarders) and/or outright war (for the war minded civs) but mainly by trying to get their own victory first.
Why is "gangpile the leader" a good design choice?
This is yet another reason why Civ 5 always ends up in war - because if you know that everyone is going to attack you at the end, why not just stomp them all earlier when it's easier? And once you do that, why bother do anything other than fight?
Not true Civ5 is not coded to "gang up on the leader". this whole dogpile the leader thing was not a conscious civ5 design choice. I'm yet to see a "your score is too high" diplomatic penalty for instance.
I've managed to keep DOF for entire games with selected civs and win the game without everyone declaring just because a start building space ship or utopia project etc. Othertimes I've played with a different play style and got the whole mass denounce thing and ended up going conquest.
The ai has a big list of rules as to what ticks them off, each leader has their own tolerances to how much each wrong annoys them if at all. Break the rules and the ai hates you, ai then uses diplomacy and gets other civs to join in. This is what causes the dogpiles. And the human player can do the samething to the unpopular ai civs.
Trouble is humans don't always know what these rules and have different expectations as to what is good/bad in game behaviour (like being at war with same foe is automatic friendship, and its ok to wipe out another civ if they started it, and its ok to wipe out a CS if its unalligned) are and so end up acting like they were razed by wolves (from ai point of view) and so end up getting dogpiled.
I like that the ai reacts to your in-game actions rather then having them like/hate me based on some abstract and gameable mechanic civics/relegion. Its facts on the ground that matter not philisophical differences. I want the ai to have their own goals and agenda, it make sense that they may covet my land and be jealous of my wonders etc.
Sure the system needs more work (several more patches) to get things better but I like the design philosophy - ai has a goal/agenda (victory type) and what matters most in diplomacy is in game actions of player.
Part of the problem is that the ai has too many things that tick it off and not enough ways to make amends - needs to be more give and take. The result is that there is a bias toward bad relations and little/no way to make amends.
Another part of the problem is that the rules are too opaque, needs to be more feedback as to when the player is breaking those rules. You just have no idea that an ai is reaching its breaking point as to when it may decide to denounce you.
A lot of us choose to win in other ways - like beat our fastest time at something, or win with an usual nation/victory condition choice. We don't need the artificial "difficulty" of backstabbing computer opponents. It's just a feature for people who want combat and dislike peaceful builder games, and it's one that's forced on everyone who plays by an inexperienced designer.
ai doesn't always backstab but it does have the possibility based on leader traits - much like in civ4. this is a good thing
There needs to be at least some ability for the ai to do a well timed backstab when a strategic opportunity presents itself otherwise there's no uncertainty, no risk - do USA,Europe,China,India,Russia really trust each other?
btw I tend to play a hybrid builder/war game (depending on mood and strategic situation) so builder with some wars. I seldom go for straight domination victory. I tend to fight at least one war though cause someone close by is usually a jerk.