Why are people taking affront to the ending when it purposely leaves out Jax's actual method of attaining it?
He says to dream bigger. Everyone with the argument that erasing chattel slavery in the United States would leave either the world unrecognizable or even worse off isn't even trying to imagine a scenario where things wind up being familiar, but better in every imaginable way.
Jax at this point is an immortal time god. He mentions that it takes several iterations of modifying the timeline to get to the point where he, his wife and child are still born. Going by the other fighters' endings, it probably took a near eternity to get this ideal timeline in motion.
But let's say he goes back and steers humanity towards technological breakthroughs centuries ahead of time, and leads the most powerful heads of states into designing egalitarian societies, while deterring the slave trade in Africa (and possibly everywhere else in the world) through some combination of the above.
Let's say you can adjust history to make colonialism an unattractive and costly policy for western Europe. Maybe you can find a compromise between isolationism and colonialism. Maybe the heads of the Church have a series of epiphanies that steer civilization towards brotherhood and tolerance.
What if you had infinite time, and infinite resources, to solve each of the fundamental issues with civilization that lead to the institution of slavery? Because that's what Jax has. We don't know how, precisely, he pulls it off, but he does.
In conclusion, fuck Doctor Who.