Oh, granted. Problem is, that's exactly how things worked out anyway, so we're not comparing [that happening] to [that not happening], we're comparing [that maybe happening] to [that definitely happening]. It's not a given that abolition in the late-18th century would have been as arduous, involved as much violence or had as long-lasting repercussions as abolition in the mid-19th century. Or at any rate, and this is really the point I'm getting at, not so evident that it offers much defence against the participation of many "Founding Fathers" in an institution which I can hope we all agree was an unqualified evil.
edit: Except Classical Hero, it appears, who in the spirit of true Christian charity argues that slavery was in fact a positive good, because: Classical Hero.