"The world economy is more complicated than any of us can understand and getting more so by the day."
[...]
"If economists were honest according to this piece, their advice is not worth more then that of a priest or a court astrologer."

This does not quite follow. Pretty much any ecosystem is more complicated than any of us can understand, yet scientists are able to make reasonable predictions of the effect of a single factor even if they can't describe the end state.
We never know the exact positions of atoms in any system yet we are pretty good at knowing the macro behaviours of solids and solutions.
We cannot know the precise state of any chaotic system but we manage to make pretty accurate prediction of the weather, an incredibly chaotic one.

Just because something is too complicated does not mean we can't say anything. And, to be fair, quite a few economists (those who did take real-world evidence into account) did have useful things to say. A major problem came from the fact that people in positions of power were able to buy the type of advice that they wanted.

Many economists realise that -I was at a conference at LSE and asked a few questions to the speaker on this line, and the answer was that people in power will probably not change the way they view the world, but he was hoping that his work would have some influence over next generations of decision-makers.

One the one hand I fear he is deluded -there will always be enough economists ready to be bought to sprout wealth-serving nonsense. On the other hand, IMF and OECD have finally changed their line somewhat. Belatedly and imperfectly indeed, but recognisably so. So maybe they do have a bit of very long term influence.
Although it is yet to translate into political action, so whatever influence they might have is depressingly slow, but not just because economists want to retreat and hide. Krugman and Wren-Lewis have been quite vocal in expressing their frustration that policy was in total contradiction to what economics suggested.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi