Not quite. Their other option was to win their independence by force of arms.
At which they very nearly succeeded.
You are correct that Union victory was not preordained, given their dearth of effective military leaders.
General McClelland comes to mind.
Indeed, had the slave-power declared secession under a President Buchanan type, it may well have happened relatively peacefully.
But of course, they had no reason to secede under Buchanan.
Against expectations, Lincoln eventually proved a capable war-president, and found effective generals to lead the effort.
Yes, I'll grant you that any number of "what-ifs" might have changed the outcome, but one "what-if" would not change, short of Lincoln's assassination early in the war: Lincoln was not going to give up the fight unless completely defeated, and given the relative strength of the northern military-economy, that seems most unlikely.
Indeed, unlikely enough that I'd call Union victory more-or-less inevitable.
So, the Confederacy's fate was sealed because Lincoln (like Washington & FDR) was an "Unconditional Surrender" kind of guy.