“I think we can agree that it had nothing to do with caring about the slaves.”
Nope. The anti-slavery movement had been going on in England for 100 years. Uncle Tom’s Cabin came out in the 1840s and gavanized the North against slavery. There is a strong Christian reason to ban it.
“I was thinking that 4 million sudden extra bodies in the poor southern economy would have the same effect as high immigration, keeping the wages of the poorest workers suppressed and it would keep the South from developing economically, while the North would benefit from their ownership of Southern industries.”
Unemployment wasn’t a problem. Labor shortages were the norm. We had more land and more jobs than people. We had very open borders at that time.
As it was, many free slaves stayed working on their former masters’lands as sharecroppers or hired hands.