End slavery? American slaveholders were instead looking to expand "the peculiar institution" not just further west but south, into Mexico, Central America, and South America, by military conquest and colonization. This is well documented. It took the Civil War and the collapse of the Southern economy to stop those plans.

The idea that the US was on a path toward gradual elimination of slavery is not supported by evidence. The agricultural economies of the slave states were perfectly functional, at least as far as the slave owners were concerned, and they held all political power.

Cotton, and to a lesser extent sugar, were enormously profitable industries with global reach. And slaves made those industries work in the South. It took a revolutionary war to overturn that paradigm, and as far as most historians can see only war would have ever made it happen.

The idea that slavery was dying out is historically absurd. It was not and it would not have as long as people were making big money off of it.