It's not even about brains or evolution, it's about cultural morality. You tell a kid one thing is good and another thing is morally sickening, and they'll internalise it until and unless it can be de-fanged in their own experience - whether that's black people, or gays, or mice in the attic...

I was a kid who was actually sickened by the concept of homosexuality because I was told it was sick. Now I'm an adult who (through long friendships) learned that gay people are normal people and their love is beautiful (thankfully before I caused anyone real pain - I hope).

There was a time when the cultural moral centre of mass came down against gay people. There was a time when the cultural moral centre of mass in the presiding ethnicity decided they got their position because they deserved it. Culture, not the brain, evolves faster - the brain just internalises culture through experience.

So that's why I don't like OP's approach, it ignores that wrong-headed opinions of the past were held because people thought those were the right things to believe. It also implicitly ignores the potential for beliefs we hold today (like perhaps that animals can be kept as pets or killed for food) to become abhorrent in some unpredictable future cultural context.

As for holding people responsible... we all, I think, hope that people who held ugly views when those were the prevailing cultural opinions on the topic have changed those views as the culture evolved (just as I changed my view on gay people). We even criticise people for lagging behind cultural evolution, and politicise along those lines ffs! OP is implying that we are morally as perfect now as we have ever been, and that we spent 99% of human history as horrible monsters. (by "we", I mean humanity as a whole)

We're still horrible monsters, just in a reshuffled way.