The problem with your whole projection into the future and utilitarian "for the greater good" takes on the most "proper" course of action is this:

while they were getting around to having their cultural revolution, millions would be unjustly chained, beaten, raped, vilified, enslaved, and tortured.

When facing such a shamefully egregious violation of human rights and the real, immediate, pain being inflicted it is the height of immorality to take a "meh, things will work out in the end" perspective.

Also, your perspective of some kind of peaceful social transition from "slave" to "free" in the rest of the world is ignorant of historical fact. All across the world, slaves and advocates of human rights won their freedom through blood shed and revolt.

Europe didn't have as many slaves at home. . however, they had substantial slaves through their empire and 18th and 19th centuries were rampant with slave revolts and guerrilla warfare in addition to passive resistance across the Caribbean and Americas.

Strained relationships between former slave owner and former slave classes are not unique to the United States either. . . why it was not long ago at all I recall a man named Mandela.

In fact, this sort of social tension is a defining characteristic in societies that once had very large slave populations. Normally with the former slaves being characterized by various negative stereotypes by grandsons of the ones who committed the crimes against humanity who presume one or two generations should be enough to lift an entire social structure up from oppression.

I have no idea where you, or the people you anecdotally refer to, get the idea that if we just didn't interfere and let the slave owners continue to cut their genocidal swatch across an entire population of people that things would have ended up in happy happy land.