The civil war itself destroyed the south’s market for cotton. The number of slaves that fled was ever increasing and the war made it even easier.

If the north and south were separated they would have continued to come north but would then be asylum seekers.

The north of the south would have been the main producers of economic growth as mineral exports from that region exploded after the civil war. Based on this alone it’s not certain the confederacy would have actually collapsed.

It would take someone with deep historical knowledge of that era to make any realistic predictions of what would have happened.

For instance, the likelihood of the confederate states not further splittering isnt known. And then there are issues such as if the west coast or other regions would do attempt the same break from the union.

There are all sorts of trade imbalances that would be in play. But it’s hardly an idle thought experiment. There are simply too many pieces.