No one is saying that he didn't do anything positive. But his negatives are costing the Rams games. All else being equal, if he doesn't turn the ball over the Rams win. And it's not just that he's turning it over, it's when and where. His turnovers are either deep in our own territory or they're forced passes on early downs that didn't need to be thrown.
Yes, teams have overcome QB turnovers but Stafford was brought here to shore up that problem. To be a positive difference maker, not someone the defense has to overcome.