By that logic though, would it really matter if Cornell was replacing JMS instead of Roberson? Most likely he'd still suffer just the same. And it's kind of unfair to dismiss Roberson's talent level when he really hasn't gotten a chance to show his talent on the bigger books, especially since his other work has been fairly good.
And as I've said, we do know they'll be finishing the arcs and stuff, but past that, it's entirely likely that they'll do their own thing, even if they have JMS' notes. Williams said he had some of Rucka's notes for Batwoman, but he'd be doing his own thing, even if incorporating some of those ideas into his own run. I think we need to wait and see before we start dismissing people outright