The real fallacy comes from the idea that 2 pilots on the flight deck can even prevent a determined nutcase at the controls from crashing the plane.
The two-crew rule was mainly in place in the US to facilitate flight deck crew identification upon return to the cockpit after a break. For aircraft without video surveillance at the door, there was a need for somebody to confirm the person at the door was alone and authorized. You don't want the only guy left at the controls to have to get up out of his seat to accomplish this. The US does have differing O2 rules when above FL250 compared to the EU and other parts of the world, and this procedure of using the cabin crew was in line with that regulation.
For the reasoning behind a two-crew requirement post-Germanwings, we only have to look at the nervous traveling public and their accommodating airline executives to find our answer. No airline wants to appear less safe than the others and the idea of a sense of security - however false it might be as in the case of a two-crew cockpit scenario preventing a deliberate downing of the aircraft - is appealing to both passengers and airline executives.