A lot of your QB's that played "all 4 years" of college entered the NFL at the age of 22. Taysom is already 23, and will be going on 24 by the next NFL draft. Age was a big knock against John Beck.
If Taysom was a 20 or 21 year old junior, I'd be more inclined to side with you. He's not, and he's definitely taking that into consideration. He's already older than the current ages of every QB taken in the last draft, and maybe even the 2013 NFL draft as well. Whether or not you want to think this is important is irrelevant. NFL scouts do, and their opinion is what matters.
Cam Newton left as a junior, completing 66% of his passes. Manziel just left as a sophomore completing 70%, but Hill is bigger and stronger than Manziel. Aaron Rodgers left after his junior season, completing 66%. Taysom is the same height but more athletic than Rodgers.
Again, if Taysom wasn't an RM, I'd agree with you. But he is, and if he can complete at least 65% of his passes, he will have a strong incentive to go ahead and go to the NFL.