The problem is if the Confederates had won, the government would have been in the business of supporting slavery which involves government big and intrusive. And it was not long ago that Southern govenment was so big it mandated who could drink out of certain water fountains.
Slavery was dying (even in the south) well before the Civil War.
Slaves were illegal to import in America for decades.
Only a small percentage of Southern whites even owned slaves.
Slavery was becoming an uneconomic model due to industrialization, standardization and vast increases in productivity, especially in agriculture.
It would have died naturally. Without the need for nearly a million Americans to die.
The slave states did write the national government into the business of supporting slavery.
Let's name the slave states: New York, New Jersey, New Hampshire, Connecticut, Massachusetts, Pennsylvania, Rhode Island, Delaware, and Maryland.
Also, Virginia, North and South Carolina, and Georgia were slave states.
The US Federal government was already in the business of supporting slavery. The Union was a union of legal slavery for "four score and seven years", and if the South had not left the Union, the Union would have remained a legal slavery union for another "four score and seven years."
The war gave them an excuse to ignore the actual law and do what they preferred.