Like most of you kindly, repeatedly point out, we don't know if Tanner is better.
But what we do know is that Taysom has struggled to perform in games. Yes, the receivers drop 2-3 catchable balls per game. Yes, the o-line has had its struggles. But yes, Taysom has struggled for 5 years at going through progressions and putting touch on the ball and not just sitting in the pocket and hitching.
I don't think most of us that want Tanner to play "know" he's going to do better. But his performance last year, the close battle to name a starter this year, even his 'hype' coming out of college, and Taysom's under-performance this season means that Tanner deserves the SHOT to prove what he can do.