>Thus the question remains: What kind of policy one exactly wants to base on 'human's irrationality'. And whether it is good to base policy on irrational behavior.

That isn't what people are saying. What people are saying is that Capitalism is predicated upon the notion that people are at least mostly rational actors who act in self-interest, and so a free market will give people what they want. The criticism of Capitalism is that humans are not mostly rational, and regularly don't act in self-interest, and this leads to large amounts of human suffering.

People like Piketty aren't looking to eliminate the disparity between "irrational actors" and "rational actors"^, or to increase the amount of "rational" behaviour, but to eliminate the human suffering that Capitalism is supposed to eliminate if it worked as claimed.

^ Which is not a synonym for "wealthy". What Piketty is talking about is wealth accumulating to the point where the skilled and hard working don't have upward mobility. The hereditarily wealthy don't need to be particularly "rational actors", they just need to not spend their wealth faster than it accumulates.

P.S. Even "rational actors" suffer losses caused by the irrationality of the market.