That's not even close to true. There are ~30 million slaves worldwide today [1]. I can't find a concise worldwide total for around the time most people imagine is the peak of slavery, at least in the US (1860?), but that was "only" less than 4 million [2].
Whatever point was being made about slavery as an example in this conversation IMO points exactly the opposite way the GP intended: that slavery is a "natural" state for humans to end up in, and it requires continuous effort to prevent it. Just like war.
1. https://en.wikipedia.org/wiki/Slavery_in_the_21st_century#St...
So currently 30 million slaves out of a 7 billion population is under a half a percent, compared to 4 million slaves in the us out of a US population of 30 million is almost 7 percent.
I'd say that's an improvement
1. https://www.theguardian.com/global-development/2015/jun/18/5...
In the US, slavery was eradicated literally by the sword.
The real genius of the Founding Fathers of the US was their distrust of human nature (and consequent aversion to centralized power).