Taysom, in his Junior season where he had a few games and his first half at Nebraska, was a better QB than Tanner was last year.
Who will end up better? That's still up for a debate.
And I strongly disagree that this will mess with Tanner's progression. He still has 2 full years in the program after Taysom leaves. That will be 3 years as a starter. Most great QBs at major programs don't get that long.