It's not that he *can't* it's that he likely won't. He's not a 11th grade QB - he *can* have success, but everything indicates he's not going to do so. Whether it's the production on a stacked team, or the years of futility, the inability to ever win a starting job, his limited physical talent, his game manager style, he's always going to have to be more of a game manager than a gamebreaker. That's not much of a success. Watching him last year it was like everything had to be lined up for him to succeed. He didn't look like a guy who was making a big impact; he looked like a guy taking advantage of a good situation (in the good way, not conniving but using the assets around him) and not making other people better. Typically speaking, at this point in your career you are who you are.
I know it's your opinion he will do well. And I don't take issue with that - reasonable people can agree, disagree, etc.