The most important statistical idea of the past 50 years is the same as the most important statistical idea of the 50 years before that:
"Due to reduced superstition, better education, and general awareness & progress, humans who are neither meticulous statistics experts, nor working in very constrained & repetitive circumstances, will understand and apply statistics more objectively and correctly than they generally have in the past."
We barely have any concept of how education has penetrated our society, to a far greater degree, now than fifty years ago.
We as a population are FAR more educated, far less ignorant, and generally speaking extraordinarily better off in our daily lives than in the past.
It's only because the phenomenon of social media, which has crowd sourced ignorance and hurled it in front of our eyes, that we perceive we're in an unenlighted age/trajectory.
"Due to reduced superstition, better education, and general awareness & progress, humans who are neither meticulous statistics experts, nor working in very constrained & repetitive circumstances, will understand and apply statistics more objectively and correctly than they generally have in the past."
Sadly, this idea is still wrong.