A reasonable point.
Except that abolitionists weren’t using slaves, so they weren’t trying to make southerners stop using them while continuing to use them themselves.
The problem was not that slavery would be easy to get rid of. Every reasonable person agreed it would not be easy. Lincoln stated many times that it would be very difficult.
However, the Republican position was simply that a great evil doesn’t become good because it’s difficult to get rid of. First agree that it is an evil, and then we can discuss ways of reducing and eventually eliminating it.
But southerners, increasingly over the 19th century and very nearly unanimously by 1860, refused to agree slavery was an evil. They insisted it was a positive good and should be continued forever and spread into new areas.
That’s what Republicans objected to and to support which the original seven states seceded.
Not anymore, they had already made their money off of them. This aspect of their existence is analogous to the Hydro electric power I mentioned. People who no longer need fossil fuels want to condemn those who still use them. Same thing with Slaves.
The problem was not that slavery would be easy to get rid of. Every reasonable person agreed it would not be easy. Lincoln stated many times that it would be very difficult.
Nobody wants to cut their income stream, even if it is derived in an ill gotten manner. Yes, convincing people that their income stream needs to go, and that they don't get to feed themselves off of the work of others, is a very difficult sale. We've been trying to get Washington D.C. to do it, but it hasn't been working so far. If anything the problem is getting worse.
That’s what Republicans objected to and to support which the original seven states seceded.
Granting that assertion, it doesn't impinge upon their right to leave as expressed by the Declaration of Independence. If they practiced polygamy, incest and gay marriage, they still had a right to leave, and people didn't have a right to stop them simply because they disagreed with their morality.
My point is that the most important principle involved here is not the morality of those seeking to exercise a right, but that their rights were violated by using their bad morality as an excuse for depriving them of their rights.
Their rights do not hinge upon their morality. They can be immoral, yet still be entitled to their rights.
The South Joined with the US voluntarily. As with the manner it was joined, so should it have been allowed to leave.