Who is to say that slaves wouldn’t have been kept to run the machines? By your logic, if tech advances would lead to less slaves then the cotton gin should have ushered in the end of slavery.
The Irish got the dangerous jobs. You could pay send the Mrs. a week's pay when they got killed, and hire another cheap. Slaves represented a substantial investment and were not allowed to do the really dangerous stuff for fear of loss.
If it takes 40 slaves and 20 mules to manually plow the field. VS 1 slave to run a tractor.
What would they do with the 39 extra slaves?
Give em section 8 housing, food stamps, cell phones, car allowances, SSI disability for having inbred disabled offspring baby mamma bling, and order em to operate voting machines and vote D?
I see yer point.