No they weren't. God had already hemmed it in for them. Slavery couldn't grow anywhere in the territories because the climate wouldn't allow the creation of plantations.
Slavery was stuck in the South and couldn't get beyond it even if they wanted it to.
Given a few more decades, it likely would have dissipated in the South too.
Slaves could have been used for other purposes than picking cotton, like mining and other things. If the South had stayed in the Union, slavery might have lasted till the mide early 20th Century.