Also, technology was going to make slavery obsolete anyway.
Yes, but it was going to take awhile. If I recall properly, the first practical machine for picking cotton didn't arrive until the 1940s. Social pressure and exhaustion of the cotton fields would have likely ended it before then.
Plantation slavery was really impractical within the USA in anyplace outside of the South. The Western states couldn't grow cotton, and neither could the Southern states (without irrigation) west of Texas.