The hindsight is great with this one.
Tannehill showed the makings of an elite prospect in college with strong arm talent, timely throws on outside routes, and solid stats against a tough conference in the SEC.
Where he lacked, however, was in regards to his ability to be a fiery leader (not necessary for a QB, but many of the great quarterbacks have that) and his performance in games with big implications.
Much like Miami, when the season is on the line, Tannehill tenses up and doesn't play his best. When his job is challenged, he does enough to keep it. When he's relaxed and its just a plain ol' regular season game, he plays well.
But his deficiencies in college were the same deficiencies he has in the pros. You can coach those out of a player (see Goff in his first year with Fisher and then his massive leap under McVay). Tannehill did not get that benefit. And old habits die hard.
Sucks for him, sucks for us, but **** happens. In the end, it's not his fault he was put into some ****ty situations, but at the same time, many players have become self-made. Tannehill rode on the coattails of his coaches and wasn't the player to transcend that.