Slott's FF and IM are both doing very well. And the habit of some fans of trying to denigrate or wave off Slott's success by saying that his success hasn't been earned because, well, it's just that the characters were already popular, is flatly ridiculous. Many writers over the years have been given the chance to write big characters at both DC and Marvel only to botch the job. So just because you're writing Spider-Man or the FF, that doesn't automatically translate into success. It's not an easy slam dunk. You have to do the work, you have to keep the fanbase engaged.
Slott's done that, time and again. You don't have the career that he's had and the sustained success that he's had without consistently delivering the goods. And that "Silver Surfer thing he did" won him and Mike Allred an Eisner Award. He's done more than ok for himself, you know? Better than most people who want to make it as a writer will ever do.