While I can't argue with the fact that they did not secede in the 1820's and 30's over the issues they felt were worth seceding over, those issues did exist.
Again, would they have seceded over ONLY slavery, if every other issue were not on the table prior? How many angels can dance on the head of a pin? Both of those answers are equally easy to prove.
I'm reading you trying to make a moral equivalence, that going 56 in a 55 and going 55,000 in a 55 is BOTH speeding. Further, the way I read you is that the threat to slavery was the last straw meaning that there was no threat to it and then, with Lincoln, there was. That's not the case. There was a LOT of negotiation from the get go about slavery and, IIRC, Massachusetts wanted to secede in the 1820's to disassociate themselves FROM slavery.
If anything, slavery was THE issue from DAY ONE and all that other stuff were the rain drops.
Look. I am a native Marylander and that means I grew up with Northern and Southern sympathies. I GET both sides of the argument. White supremacy and equality. Orderly society v. a more free for all society. I am just as fond of Pat Cleburne as I am of Phil Sheridan and, like any real American, I get the ENORMOUS reverence for Robert E. Lee. I am torn, deeply, when I tour Gettsyburg or Antietam, hallowed ground I am lucky enough to live so near to. I live on ground that most of the Eastern heros, known and unknown, walk or rode over numerous times. The pull of states rights is as strong in me as the powerful pride of union. A big part of me thinks this is so much nit picking over flags just to avoid talking about mental illness. But, I also get that if my great great grandfather worked under threat of the whip in 1850 instead of threat of starvation when he came here of his own choosing I'd likely have a different perspective when I see the stars and bars. It has always inspired awe in me, the men who would pick it up from the bloody hands of the last man killed carrying it, KNOWING they'd likely soon be dead, fighting for their beliefs. Yet I am also inspired by the stars and stripes and men who did the exact same thing for union.
It's easy to say 'slavery is over' when you never were one. I made the point the other day, my grandfather, born and raised in Georgia, passed at 92. He was born in 1923. He KNEW people who were slaves. He probably knew people who owned them and THOSE people know people who were around in 1776. It was a long time ago but not THAT long.