Earlier quoted context omitted.
Didn’t the Southern States do exactly this? And formed the Confederate States of America. Where did that end up?
They had a civil rights issue (slavery) the remainder of the states could not ignore. It is another false equivalency as the PRC has never ruled over Taiwan. And the CSA did not enfranchise their populace fairly. Any such referendum was not an expression of the will of the people but merely the slave holding classes.
The union was forced as much by economics as it was by human rights, unless all the northern states would also undo their abolitionist views.