Keynes on why classical economics exerted the influence it did, from Chapter III of the General Theory:
That it reached conclusions quite different from what the ordinary uninstructed person would expect, added, i suppose, to its intellectual prestige. That its teaching, translated into practice, was austere and often unpalatable, lent it virtue. That it was adapted to carry a vast and consistent logical superstructure, gave it beauty. That it could explain must social injustice and apparent cruelty as an inevitable incident in the scheme of progress, and the attempt to change such things as likely on the whole to do more harm than good, commended it to authority. That it afforded a measure of justification to the free activities of the individual capitalist, attracted to it the support of the dominant social force behind authority.
Keynes combined a recognition that the success or failure of an idea could not be divorced from questions of power, with an optimism that good ideas, in the end, win out. I do not share the full measure of his optimism.