The point isn't that the entrenched slaving elite would have said "Oh gosh gee willikers, I guess it's time to give up slavery!", it's that counter-elites whose power did not rest on slavery, and whose power was often challenged by slavery, would arise, and eradicate slavery in much the same way as it had been eradicated in the North - by the withering of the economic and political pull of slaver elites until they could no longer meaningfully challenge abolitionist control of the legislature - and the same way that it was abolished in most of Europe.