Slavery was present for hundreds or thousands of years. It was also obviously morally wrong for the entirety of it's existence. It's decline in the western world was relatively quick compared to the duration of it's existence. This decline came about as the western world became rich enough that eliminating the suffering of slaves was worth the inconvenience of replacing their labor. This change of material conditions gave enough cultural leeway for passive conformists to embrace legislative change.
It is not obvious that the Stanford prison experiment is a complete fraud. Even with it's flaws it suggests that people are much much more likely to engage in immoral behavior when an authority figure endorses it. Historical atrocities confirm this.
I don't think there's a productive way to argue about the cancel culture point. Data supporting which side is "winning" the cancel culture war is too cherry-pickable. The only ground I can stand on is that people such as Stephen Pinker getting cancelled is obviously ridiculous.
I do not think that the personality traits discussed are superficial. Other posters have provided more evidence, especially regarding openness and conscientiousness, that I speculated on earlier. I do not think that the purpose of PG's essay is to flatter himself.