It is highly unlikely that the Constitution would have become effective without the slavery compromises. If it had, the United States would only have been comprised of non-slave states in the north, and most likely would not have survived. Southern delegates to the Convention were aware that slavery had been prohibited in the Northwest Territories under the Confederation Government and insisted that the right to own slaves must be protected. They were so insistent that on several occasions they threatened to walk out of the Convention, an action which would have severely compromised the ability of the Convention to create a cohesive union. Previously, at the Second Continental Congress, Jefferson's original draft of the Declaration of Independence listed the slave trade as one of the several offenses of George III; however Southern delegates to the Congress insisted that the offending provision be removed.
Similarly, since members of the House of Representatives and Electoral Votes were determined by population, Southern delegates had no intention of seeing their position in the new government diluted, and once again threatened to leave. The slavery compromises were thus necessary for the constitution as written to become effective.
An indication of the seriousness of the slavery debate is the fact that the "s" word is not mentioned in the Constitution before the Thirteenth Amendment which ended it. The three-fifths clause speaks only of "all other persons." The provision to end the slave trade is even more carefully worded:
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight.
Many compromises were necessary for the U.S. Constitution to be agreed upon by the 13 colonies. One of those compromses, known as the "three-fifths" compromise, dealt with slavery. After resolving the disagreement on representation (big states vs. small states) and settling on a bicameral Congress, the next big question was how to determine population. Of course the southern states wanted to count their slaves because that would give them a huge advantage in the House of Representatives over the north, which didn't have as many slaves. The northerners, of course, thought that was a bit disingenous. After all, the southerners treated the slaves worse than their horses. A compromise was wrestled. In addition to the "Three-fifths Compromise", conventioneers also agreed to to give Congress the power to ban the slave trade after 1808. While neither the north nor the south received all it wanted, they each got some of their desires.