OK, but that was sort-of my point. The more outrage, the less you need to really think about things.

"err, yes, popular anti-slavery movements played an important role in the abolition of slavery"

That is a chicken-and-egg question. Why did those mass movements only emerge at the time of the Industrial Revolution, and why did they emerge first in places that were influenced by the Industrial Revolution the earliest, while other places (Russia, the Ottoman Empire, the Qing Empire) only followed suit after their own industrialization began?

I don't think the arrow of causality is so simple here. A hypothetical society that abolished slavery, serfdom etc. in the 15th century could easily prove non-viable against its slavery-powered foes, which had more brute force at their disposal. By 1820, the situation was very much turning around and it was the modern, personally freer societies that were more effective in commerce and at war.

Notably, even though Victorian Britain was very anti-slavery, starting with the monarch herself, it had no moral qualms against subjugating a quarter of humanity in another form of submission. Which tells me that it was less about morality (equality) and more about practicality of the situation.