Sharkonabicycle
Bipedal Sea Dog
The things you describe say he's not likely to come in and start in year 1, or be successful while doing so. It does not say he can't make it.
Does he have the arm, does he have the ability to read defenses, does he know how to anticipate when a receiver will be open before they are so he can get a throw out.
He can learn how to read and throw from the pocket, under center, etc. The rest of those skills you either have or you don't. He does have an arm, but those other things aren't really well known at least at NFL levels. He didn't have to read as much of defenses at Oregon as he will in the NFL and his passing game was hitting the speedsters that were wide open. He wasn't asked to anticipate and make throws before the route breaks.
It's not that he is bad at those things, but he wasn't asked to do them.
IF Mariota makes it as a starter I think it takes at least 3 years and a fair amount of growing pains to get there.
IMO they didn't draft him #2 overall to sit on the bench and as soon as the Titans start losing some games the fanbase will pressure.
I have seen VERY few (if any) QBs have poor/unknown pocket presence and zero shown ability to progress through receivers and run a spread offense (or no huddle one) translate well to the NFL.