Of those that disagreed with the idea that slavery benefited mankind, yours is at least well thought out. There's obviously a counter current running against progress simply because people generally work as hard as they have to, but aren't enthusiastic about working harder.
It requires leisure to have the time to learn the knowledge already known to a culture, plus the time to develop and create new ideas. Leisure develops gradually, just as most things have.
Domesticating animals creates a little leisure for a lot of humans.
Private property and enslaving captives creates a lot of leisure (and prosperity) for a few that can spread that leisure out as they desire. Some of those gaining a lifetime of leisure spend their whole life learning, teaching, and developing new stuff that please the property owner that provided the academics their life of leisure.
Leisure from working is what also gives a person time to devoting their life to doing nothing but trading objects created by other people's labors. Trade was the main method of spreading knowledge between cultures prior to the printing press and the industrial revolution. (Trade even more so than kings or religions attempting to spread their influence over a greater area).
And, yes, the industrial revolution spread leisure to even more people. Lots of people have nothing but leisure time, which they spend designing new products to be bought by other members of the leisure class, since those furthest removed from physical labor always seem to have the most property, money, and other resources. Other people have so much leisure time that they do nothing all day except keep track of how much money the richest of the leisure class are earning. Other people have so much leisure that they spend their time teaching other folks how to join the leisure class.
Slavery is a little like the debate over the disparity of income in capitalism, except a lot more extreme. It eventually raised the lifestyle of everyone (even if only raised the lifestyle of a few immediately), but the road to progress wasn't very fair to the folks at the bottom of the totem pole - especially since any benefits to their kind were likely to come centuries after their own life ended.