The number of slaves in the United States increased about 15% (3.5 million to 4.0 million) from 1850 to 1860. The machinery that would have serious undermined the use of agricultural slave labor was not developed for another 80 years.
But slavery was a generation away from dying via economic changes. It died in Brazil without a war as turning workers into wage slaves became cheaper than maintaining slaves and the regulatory laws govering their proper upkeep (to avoid slave rebellions).