[jeffersondem]

“Noooo… 1787 “demarcates” the time when most Northerners began opposing slavery in practice as well as theory.”

That is an interesting comment.

Every northern state, including the Keystone state, voted to enshrine slavery into the Constitution of the United States.

But the northern states did not enshrine slavery into the Constitution gratuitously: they had good reason. It was thought to be in their economic and political best self interest.

[BroJoeK]

jeffersondem: "Every northern state, including the Keystone state, voted to enshrine slavery into the Constitution of the United States."

Only after some Southerners threatened "no Union" if they refused.
At the same time those Northerners were passing laws to abolish slavery in their own states.

jeffersondem: "But the northern states did not enshrine slavery into the Constitution gratuitously: they had good reason.
It was thought to be in their economic and political best self interest."

A typical Lost Cause Marxist explanation.
If you could go back to 1787 and ask them directly, here's what they'd say:

"We did it... in order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defense, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity."

If you then asked them about their "economic and political best self interest", they would look at you quizzically and respond, "You mean promote the general Welfare and insure domestic Tranquility, don't you?"

If you pressed them, saying, "but isn't it all just to line your own pockets with Federal money?" they might invite you to duel, or just remind you that of the 56 who signed the Declaration of Independence, over half lost their lives and/or fortunes as a result.

"We mutually pledge to each other our lives, our fortunes, and our sacred honor."

Over half lost their lives or fortunes, but none their sacred honor.

It would take the later "genius" of a Karl Marx combined with the bitterness of Lost Causers to see in all that mere "economic and political self-interest."