In a world governed solely by memetic evolution, a world where reason doesn't exist, unpopular ideas hurt each other by association. It's possible for new ideas to be better than existing ideas, but all new ideas suffer a "weirdness" fitness penalty. 1/3
What if there were cults that once ran our world and systematically eradicated the capacity for individual rationality from its host population? And you also have to deal with the firewalls that each successful ideology builds into the minds of its disciples?