There wouldn't have been any need to secede. The country would never have formed. Just that simple. The southern colonies made up (DE, MD, GA, NC, SC, VA) made up almost half the colonies, they would simply not have ratified the Constitution.
It is easy to misunderstand the reality of slavery, based on our well founded disgust at the institution, for those who held slaves. Slaves were wealth, most of the net worth of the southern colonies was in slaves. It is rather horrific to consider it, but asking a slaveholder to free his slave(s) without compensation was like asking a northern ship owner to give his ship away, a dairly farmer his herd, a homeowner his house without any compensation. It was almost literally like asking someone to go from wealth (and ALL the Founders were men of property, both North and South) to poverty at the stroke of a pen. Even a phased emancipation would have represented a steady destruction of wealth that had been built-up over generations. The number and value of slaves was lower than it was to reach by the start of the ACW (when some estimates put the wealth represented by slaves to be as high as 70% of the total wealth of the slave States) but it was still enormous, even before one considers the labor value slaves represented to those who possessed them.
The theory that the Southern States were too weak to secede is also rather odd. By far the wealthiest and most powerful colony was Virginia, without Virginia the U.S. never happens.
The harsh reality is that the only way slavery ends in the United States is at the point of a bayonet.