Can't both be true?
Book does have a ceiling. He has never been particularly much of a deep ball guy and seems to struggle to hit smaller receivers on the outside (Finke works mostly in the middle of the field). He just doesn't have a cannon for an arm. He's not Trevor Lawrence and never will be.
But his ceiling is clearly higher than the Michigan game, or much of what he showed the first half of the season. And if he cleans up his act in the pocket, as he has the last couple of games, he can be quite effective. He also can run, and he doesn't throw dumb picks. Couple that with a good defense and a more effective running game and, well, we went 12-0 last year...
I don't know. I want to see what PJ can do with full command of the offense. But if Book plays the rest of the season like he has the last couple of weeks I'm not going to argue against starting him next year.