“If I’d lived back then, I would be so up in arms about slavery…”
No you wouldn’t. You would have said that slavery was the best thing for the Africans, pointing to the comparison of how they lived in Africa versus how they live in America. You would have said that most slave holders treat their servants very kindly, and that most Blacks weren’t suited to anything better anyway. You would have said that slavery was, if not a good thing in itself, yet the best thing we are capable of right now and anyway there’s no practical alternative.
The same goes for any other injustice that you look back and shake your head over. You wouldn’t have been a feminist in the 18th century, or an anti-Imperialist in the 19th, or anti-fascist in the 20th. You would have been entirely in favor of everything that you now look back and loathe.
I know this because ‘look back in loathing’ is itself the equivalent to these things. Shaking your head over the injustices of the past is the default attitude of today, just as being pro-slavery was the default attitude of the antebellum South. Had you lived there, I have no doubt whatsoever you would fall into the default attitude of that time, just as you fall into the default attitude of today.
If you want to convince me otherwise, then you would need to actually question the injustices, abuses, and idiotic ideas of today: the ones that might actually cost you something to question. Like ‘does a disease that is only seriously threatening to a small portion of the population warrant the wholesale suspension of individual rights?’ or ‘maybe contempt for the domestic sphere hasn’t been the best thing for either women or society.’
If not, then your self-insert fantasy about the past is quite simply irrelevant.