I tend to agree re: its hard to imagine slavery continuing into the 20th Century, but I think its worthwhile looking at Brazil and how it abolished slavery. Slavery was ultimately abolished in Brazil in 1888, but it did not have the Federalism and a Constitution that protected slavery like we had in the US. So all it took was passage by a democratic majority and consent of the sovereign. And in fact, the government was overthrown the following year in a military coup that most blame on the anger of the large plantation owners over abolitionism. So even in 1889 there were significant pockets of support for slavery in a fairly modern agrarian society, much like the US south was at the time.That reasoning is compelling to support any notion that slavery was not simply going to disappear overnight in the requisite number of those slave states.
Nevertheless, personally, still found it difficult to fathom how an anachronistic slave labor system could continue into the modern era of the 20th. century in such a leading industrialized and enlightened society, like the US.
But your argument remains sound and plausible.
If a democratic majority could abolish slavery in this country, I do think it likely would have been outlawed sometime in the 1860's or 1870's, much like what happened in Brazil. But the Constitutional protections in the US create an impediment that abolitionists in Brazil did not have. That is why I think 1890 is probably about the earliest it could have been abolished, and likely some time after that, given how long getting Constitutional amendments through the entire process usually takes.