“But there were pockets of the North on the eve of the Revolution where slaves played key roles in the economic and social order:”
I don’t think anyone is arguing there wasn’t ever slavery in the North. The difference is that by the 1804 all had been set free.
Slavery in the South was not profitable and was based mostly on culture. A field hand slave cost about 1,800 dollars, which does not include cost of feeding or housing. A plantation owner that owned 20 field hands probably lost 10 percent to death or runaways bringing the actual cost of a slave to about 2,000 dollars. The daily wage for a free farm worker in those days was about 1 dollar a day. If you subtract the non growing or harvesting days it would cost a plantation owner about 150 dollars a year to employ a farm workers instead of slaves. Given that a slave likely did not work any harder than he absolutely had to, the free farm worker would also have to be more productive.
So, a plantation owner was tying up 2,000 dollars in capital for labor that would cost him about 150 dollars a year for free labor and not require any capital investment. Much of this was why the south was so poor in capital and depended on the north for most industry, causing a lot of the sectionalism that separated north from south.
Plantation owners argued that slavery was necessary given the need for labor to produce cotton cheaply enough to feed northern industrialists, but this was an argument formulated to justify slavery, and not the other way around. One has to believe either southern growers were either the worlds worst businessmen or had other motivations for why they used slave labor.
There were slaves in the North in New England even around the time of the civil war...very limited numbers...but still existed.