And what if the had just been allowed to secede?
They’d probably have continued to practice slavery for a long, long time.

Good question. So why weren't they allowed to secede? Do I think they had the legal right to secede? Yes. (Tabling for now the WHY for secession, this is about COULD they secede.)
So why didn't the U.S. allow the southern states to secede? I dunno. I do know that, just 85 years before that in 1776, England didn't let the U.S. secede without having to fight for it. Nor did Spain let Mexico secede in 1821. Nor did the new nation of Mexico let Texas secede in 1836. Nor did the U.S. allow the Confederate States secede beginning in late 1860.
I'm not saying it's right. I'm just saying it was the norm for this area. At least as far as I can tell. So maybe nobody should be surprised.
Lincoln was ready to move Washington DC to Kearney, Neb. had he lived. Records show it was all surveyed and set up to have it moved to have the Center of the US located in the geographical center of the Nation.
They were not allowed.
It’s settled.
Why do people feel compelled re-live this crap from 160 years ago?