It's not that he's a bad QB, he can definitely still play. It's that his general attitude on the field paired with a long losing streak and multiple injuries is making many people think that mentally and emotionally, he is finished. And it's valid speculation.
Maybe some winning will fix that, who knows. I don't know if time is going to fix a problem that goes much deeper than a change to his throwing motion.
By week 6, Cam will have had four more weeks of playing at game speed with the new throwing motion. The optimist in me says its like trying a new golf swing. Your golf score tends to get worse before it gets better.