The South never pretended it was about anything other than their own self interest
That is correct, it was in best their self interest to leave the Union to ensure that their economic system was not interfered with by the incoming Lincoln/Republican administration.
That's the propaganda we keep hearing, but in practical terms there was very little that Lincoln or the Republicans could actually do to interfere with their economic system. In real terms, the South would have seen massive increase in capital from getting out from under trade and taxation rules that benefited New York and Washington DC at their expense.
The powerful corruption cartel running things in the North would have lost huge amounts of money, and the Southern states would have gained huge amounts of money.
It was the underlying but unspoken reason why the North went to war.