Slavery could have been abolished. It would have taken a constitutional amendment (3/4s of the states).
Do the math on that. There were 15 slave states at the time that could have ended slavery in their states any time they wished, but didn't do so. So its a good assumption that none of those states would have supported an amendment to the Constitution to end slavery in the United states.
With 15 states opposing such an amendment, it would require 60 states supporting the amendment to gain a 3/4 majority. Last I checked, we didn't have 75 states then and still don't today. The amendment process was an impossibility.