I'm not sure if this post goes here, but this sub seems to be the place to get the most reliable, in depth answer for this query I've had for a while

I understand that there were moral implications, IE slavery, to the north fighting the civil war, and as I understand it seceding was considered treason by the constitution/laws, but what were the implications of allowing the south to secede long term? It seems to me this could've caused the 2 countries ( original USA and the confederacy) to adopt a political spectrum more left wing to center and right wing to center respectively, and as long as trade and immigration between the countries was still allowed, it would've caused much less political division modern day, at the cost of cutting potential military power in half. Was it based on the assumption of the 2 countries not being able to coexist and get along?