People forget that slavery was just one of many issues that were dealt with. The Civil War resulted from this stupid compromise because of the Northern states taking economic advantage of the Southern states due to their increased power in government. Slavery in the United States would have died out once improved mechanical processes came along to the extent that the cost/profit bottom lines called for it. This compromise, in essence, created a future war between the two factions as one side, the North, was given an advantage in power that they abused.
Yes to all that but at the time the compromise was necessary in order to even get the Constitution done, the future not being terribly clear to the delegates. What changed was the population of the industrializing northern states increased faster than that of the southern states.