That is not necessarily true. If the United States accepted the idea that the populations of states have a right to become independent, "and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness...", then there would have been no need for a war. States could have joined the confederacy if/when they saw it in their best interest to do so.
More like, the Union would have had to adapt to the economic conditions caused by the South's embrace of lower tariff trade(not exactly free trade, but closer to it), and engage in less protectionism, less subsidies, and less tax and spend fiscal policy.
It might have caused Washington to have more discipline than it has now.
See my post #564 above to learn why no such thing could ever happen.