Slavery was key to the generation of wealth in the South. A threat to it was a threat to the means of production in the South. If owning slaves did not generate wealth, there'd never have been any interest in the North.
Yeahbut, their own words say it was about slavery.
I don't agree that your last clause follows, btw.
You noticed the comments about slavery being blessed by their religion, and how their "beliefs and safety" were being threatened, were buried in there, too, I trust.
If secession were to be justified on economic grounds, they could have said so.