But the Founders were not explicitly fighting to protect and extend the institution of slavery.
The secessionists were.
Slavery, as you say, was not an issue in 1776. In 1861 it was.
The Union wasn't explicitly fighting to abolish it either, and barring Ft. Sumter where no one was killed during the Confederate attack, the North was the deliberate aggressor.
But the point remains, the founders did not regard the principles expressed in the Declaration of Independence as applicable to the slaves. Their own personal continuation of slavery demonstrates this conclusively.
Slavery, as you say, was not an issue in 1776. In 1861 it was.
Not to the union it wasn't. Their "principled" leader would gladly give up the slaves to get what he really wanted.
Ever since the civil war, people in the North wanted to believe the war which killed 600,000 people was fought for a noble purpose. It wasn't. That noble purpose got tacked on towards the end to justify all the bloodshed which occurred. Having suffered such heavy losses, the North would have revolted if no better rationale than forcing Southern states back into the union was not forthcoming.
Again, the abolishment of slavery did not motivate the North to fight the war, but it did give them a moral justification for getting so many people killed in the pursuit of an immoral purpose, AFTER. THE. FACT.