Suppose Lincoln had done this in '61?Bad idea. Route to national self destruction.
It wouldn’t “destroy” the union. It would simply have been smaller. The original 7 seceding states not even the states of the Upper South which seceded after Lincoln chose war ever proposed to interfere in what kind of relationship Wisconsin wished to have with Vermont or any of the other states which remained in.
If anything, letting the original 7 go in peace would have sent a clear message that the federal govt dare not grow too oppressive lest other states choose to leave. Additionally it would have meant the rapid collapse of slavery in the seceding states (and maybe even the states which still had it that remained in?) since all a slave would have had to do to gain his freedom would have been to cross the 1500 mile border. States that were no longer part of the US would not have been covered by the fugitive slave clause in the US constitution since they were no longer part of the US. Lincoln himself pointed this out repeatedly and nobody could dispute it.
We have been on the route to national self destruction for decades. And its getting worse.