Sanderson is a great writer in his OWN right. I think it will improve his sales, but also his notoriety.
His other books, as spectacular and intriguing as they are, are relatively unknown. Finishing the Wheel of time will be hard for him, but more people will read his other books, and many will shun them completely, because he "isn't robert Jordan."
He isnt', but he's doing his best, and I think he'll be wonderful