Had the south seceded they would have been able to sell their cotton without the onerous tariffs. They wouldn’t be forced to sell their cotton cheap to the north. Slavery would have ended soon.
The real question is: How long could the northern textile mills last without the cheap cotton extorted from the south? Not long.
Not surprisingly, the northern textile mills did just fine without the "cheap cotton extorted from the south".
Oh dear, where do we even start with this?
How about here: there were no tariffs on exports, none, never, ever, ever, never: none.
Is that clear now?
There were, of course, tariffs on imports, and these were intended to both provide Federal revenues supporting the Army & Navy, and to protect US manufacturers.
Everyone paid, directly or indirectly, import tariffs, North, South, East & West.
Most tariffs were paid at the major ports of entry being primarily New York, Boston, Philadelphia & Baltimore.
Very small percentages were paid by Southern ports like Charleston, Savanah or Mobile.
New Orleans was a large port for both exports and imports, but as much of its product came from, and went to, Northerners down & back up the Mississippi River.
Of course, the pro-Confederate claim has always been that only the earnings from cotton & tobacco exports paid for the imports which provided tariff duties in support of Federal revenues.
Yes, there is some truth in that, overall it's about half true: by 1860 cotton exports were half of total US exports while tobacco added another 6% or so.
However, other US exports, especially manufacturing, were rising rapidly, and the US was able to quickly adjust to the loss of Southern cotton & tobacco during the 1860s.
Now on the question of protectionist tariff rates -- these went up & down over the decades before 1860.
They reached a high around 35% under President Andrew Jackson and Vice President Calhoun, the so-called "tariff of abominations".
By 1860 US tariffs were reduced to near its all-time low of 15%, which is also the level set by the Confederate Congress.
In the mean time, many Northerners supported a higher tariff, called the Morrill Tariff originally proposed around 22%, but opposed and defeated by Southerners in Congress.
When those Southerners seceded and resigned from Congress, the Morrill Tariff passed, and was soon increased again to pay for the Civil War.
ladyjane: "They wouldn’t be forced to sell their cotton cheap to the north."
Of course, no free person was forced to sell anything to anybody, and roughly 80% of Southern cotton shipped to Europe, not the US Northeast.
By 1860 Southern cotton was the sole, best and cheapest source of cotton in the world -- that's why Southerners were so prosperous.
They were, in effect, the OPEC of cotton and that made them proud to the point of arrogant about it.
They came to believe that, like OPEC, they could cut off the supply of cotton and bring the world to its knees before them.
So that's what they did in 1861, but it failed.
Europeans quickly enough found other sources for cotton, and the South never fully recovered.
ladyjane: "Slavery would have ended soon."
The Confederacy was unlikely to end slavery so long as it remained economically viable.
How long could that be?
Well, already by 1860 many slaves were working in Southern factories, out-competing white workers.
Of course, not to the extent found in the North, but the South had manufacturing, and it was growing rapidly.
Plus, the Supreme Court's Dred-Scott decision meant there was no lawful way to prevent slaves from being put to work in Northern factories!
ladyjane: "How long could the northern textile mills last without the cheap cotton extorted from the south?"
By 1861 there was already a huge overstock of Southern cotton in both Europe and the Northern US warehouses.
This allowed enough time to bring other global sources of cotton on-line, especially India and Egypt.
So it turned out that, like OPEC in recent years, Southerners had grossly overplayed their "cotton card".