Treating what until recently they'd thought of as their country now as an enemy or as some nothing to be despoiled at will produces bad blood and an inevitably hostile reaction from the rest of the country.
If you want to learn from experience, look at recent arrangements in Canada and the UK where both sides worked together to create conditions for possible Quebec or Scottish independence.
But I suspect the point of all these arguments is precisely not to learn anything. It's to go around with the same "f-you" attitude that worked out so poorly for everyone the last go-around.
It's not the goal of independence that some people actually want. It's all the adolescent drama of slamming the door on the way out and shouting parting obscenities. It's not the actual change in one's status that's valued, but the feeling that one was right and justified all along in whatever one does.
Actually the point was that a war was not inevitable. Had Lincoln opted to negotiate vs dominate, there would have been no war. I point you to Czechoslovakia, where neither side demanded a union.
Self-determination is a very basic and fundamental right. It is also a difficult one to define and be logical/fair about. It is not at all clear that the Constitution bars a state from leaving the union. Indeed, the organization of the federal government reads like a confederation.
How do those claiming that secession is unacceptable explain the creation of West Virginia by the defenders of the sacred undividable union?