I think they are playing to win, but if a lot of them decide on a cultural victory path early on, then the game gets very passive. I just won a diplomacy victory where three of them were clearly going science, India was going cultural, and a couple of domination civs got beaten back. I think the AI was working fine in that game, even though there wasn't a ton of warmongering going on.
I agree that the complaining never ends. Personally I'm enjoying not being DoWed within the first 100 turns. If it happens every single game, then yeah maybe there is a need for slight adjustment. But good lord, people have done nothing but complain about the AI unpredictability and aggressiveness, and now that they have some measure of predictability and are more peaceful, it's not good enough. Talk about moving the goal posts.