Historical/​Rationalistic Assesment Question

Assume a highly rational actor with as much knowledge of the world as they could realistically have. Roughly what point is the ‘turning point’ in history after which they should be able to clearly realise that Western democracy is superior to European-style monarchy from a perspective of human welfare?

Clarification: By superior, I mean ‘overall superior’- that a variant of Western democracy is a better sort of system to think about when trying to make an ideal system for a country than a European-style monarchy.