All evidence to the contrary notwithstanding?
The advent of machinery was speeding up the obsolescence.
What machinery was that?
The cotton gin introduced an artificial bump in the need for manpower, but after a time, even that became redundant.
The cotton gin made slavery profitable by automating the removal of the seeds from the cotton boll. However, harvesting the cotton was a manual process and remained so till the 1940's.
Bleeding hearts just can't see that slavery was not that important an issue.
The southern leadership of the time would disagree with you.
What evidence do YOU site? The financial ledgers at that time indicated that the cost to keep a slave population was through the roof. Contrary to Disneyland fantasies about slavery, it cost money to keep them. (Oh my!) An expensive piece of property needs to be maintained if it is to be any use, and slaves are no different. The cost just to feed a slave population skyrocketed in the late '50s.
What machinery was that?
Oh, golly gee, let me think: how about the steam engine? modern thrashing machinery, milling machinery, ....Non farmers don't think of these things. What was the north MAKING if not modern machinery?
The cotton gin made slavery profitable by automating the removal of the seeds from the cotton boll.
But you already said slavery was profitable BEFORE machinery, which is it? Another point here is that it still cost the farms money to maintain the slaves.