Originally posted by Old Man Whirly!
they didn't fix everything though.
No, but their very existence and activity raises the question of how much of Hari's plan is his vision, and how much is the Second Foundation stealth reprogramming.
Think of it like this, if society is a complex AI, and the programmer predicts its behavior patterns over a five year span, lets it loose in the wild, and continually reprograms it if it strays too far from the prediction, then the prediction was inaccurate.
The Psychohistory element of Foundations was initially presented as a kind of fortune telling. The Mule pops in with "magic", something Hari could not have considered in his plan, to ruin the plan. The Second Foundation is revealed as an emergency measure against the unexpected.
If that was as far as it went, then fine. But they are very much active in shaping the plan. Yes, they have a code, but is the element of a group of psychics really necessary? Does a "benevolent guiding hand" need to exist at all?
If the answer is "yes", then why bother with Psychohistory as a plot element? Strictly speaking, the psychics could force the plan into being, but choose not to. So why don't they? What stops them from doing so?
And if The Second Foundation did not exist, could Psychohistory stand without them, assuming no further aberration like The Mule interfered? If it was only society as it was presented in the first Foundations novel, based off the Roman Empire, could it have worked?
Psychics playing "guiding hand" muddied the waters, at least from my vantage point.