When was it ever claimed that the game was fair at "Incredible"?
If you want to play the game with everything "fair" then play at "Intelligent". If you can beat that level, then you're better than the AI is at the game. At that point, you have to ask yourself, what's next? Do you just put down the game and declare victory or do you go to the next level where the AI gets advantages?
In a game where the AI doesn't cheat, you will always have a percentage of people who are better than the computer players since, they are, after all, only as good as the AI developer.
I'm pretty good at strategy games but there will always be a certain percentage of people who can beat me.
And since most of the top players were winning through diplomacy, I don't see how it's "unfair" to have the highest level AIs simply be more careful about what they trade. That they won't trade unless it is very much in their best interests.
Take a few steps back and read what you're suggesting -- you want the AI to trade more with you so that you can win. Therefore, logically, for the AI players to win their best strategy is to not trade too much with you unless it's certain you're getting a very good deal.
Besides, humans have all sorts of advantages too -- Ctrl-N, saving games, human intuition, etc. These are things unavailable to the computer players. Incredible AI level isn't meant to be fair, it's meant to be masochistic.
~SDC~
Stardock Forums
I'm pretty good at strategy games but there will always be a certain percentage of people who can beat me. 
