And what if the owners didn't want to be compensated for their slaves, but wanted to keep them instead? A gradual emancipation plan would need the support of the slave owners, billions of dollars to fund it, and a solution for what to do with all those free blacks.
I think that later on, owners would have been more conducive to the idea. And the cost would be much less than what it would cost to fight the most destructive war in US History.