> An obvious example was the decline of slavery ( a huge proportion of the population of the Roman Empire) but the descendants of slaves were not the only people who benefitted from the removal of imperial power, and heavy imperial taxes, etc.

Look at how a lot of those societies were structured. Were those people really doing "better" or are we just assuming that because of the biases our modern culture brings?

Being not a slave across the rome-middle ages boundary is like having a degree in liberal arts. It might've meant something at first but the back slide basically watered it down to nothing for a lot of people. That's why it went away. There was no point in maintaining it as an institution generally after Rome fell.

More broadly, there's a reason nobody really cared about slavery, rights, freedom, etc, etc, until the 1600s+ (i.e. the beginning of the off ramp toward industrialized societies). Prior to then so much of society was enslaved by the literal physics of the work that needed to be done to keep a roof over everyone's head and food in their stomachs that it didn't really matter. Almost nobody was in a place to exert more influence upon their life arc than the wind does upon the path of a stone thrown through the air (which is to say some but not much) so society didn't expend effort to hash out the details of something that wasn't relevant. Only once there were more surpluses the various shades of freedom become something that society could benefit from defining.