The problem is that the framers did compromise. They created a three/fifths compromise that gave Southern white property owners extra powerful votes. Why would the South give a greater compromise? Or what would the North have given in return? It's easy enough to think of compromises that would have worked if the South had agreed. But why would the South agree?
- Gained population for seats at three fifths of their slave population.
- Faced potential restrictions on the importation of slaves after 1808. Importation was banned after 1808 as a result.
Yet slavery continued for more than fifty years after that. Slavery ended when the South refused any possibility of compromise and started a war.
Things like children not being born slaves could have had perverse consequences. For example, who would feed the children? Not the slaves, who often had no money. The plantation owners would no longer gain value from the children of slaves, so many would not bother feeding the children.
I actually think that you'd be better off the other way. Let the South and North separate immediately. The Southern capital would have been in Virginia. The Northern in Philadelphia or New York City. Let the abolitionists in the North help escaped slaves. Then they wouldn't have had to have had continued compromises as they expanded to the west. States could have chosen to join either the North or the South. The North was richer with a greater population, so most states might have preferred the North (in actual history, they held up states that wanted to join the North so as to allow time for the South to find states of its own).
A lot of what I personally find most repellent about the compromises that were actually made was that they continued to go the South's way. Things like the Dred Scott decision. That was in 1857. A split would have fixed the ongoing problem. The North would have been able to be better that way. In actual history, the South used its influence to maintain its influence. That was the result of the three fifths compromise. Any other such compromise would have likely given too much to the South.
The problem is that the South had too much influence and very little of the compromises went against them. People in the North were required to help recover escaped slaves. If the North had been a separate country, that wouldn't have been so. There would have been more support for escaped slaves. The South would have needed to spend more resources on keeping their slaves. And slave owners couldn't have taken slaves north without risking losing them.
Separate, the two countries could still have had a common defense agreement. They just wouldn't have also had a common government. Perhaps that might persist to today. Or they might have united in the late nineteenth century, after the McCarthy cotton gin made slavery non-profitable and world opinion swung away.
A separation avoids the Civil War, and it allows the North to be more abolitionist. Note that the Act Prohibiting Importation of Slaves was passed by a Southern president. So they might still have passed it. Either then or under international pressure once the United Kingdom banned slavery.
Of course, a separation also risks that slavery would have lasted longer. Without the join, there would have been less reason for the South to have compromised. Perhaps they would have stayed as they were longer. Or they might have faced a blockade earlier due to international opposition to slavery. Slavery was an internal question in history. But if there were two countries, it would have been an international question.