redshirting this year. That was before any doubts about Taysom had arisen (well, at least any public doubts--whether or not the coaches were already thinking about the need to replace Taysom with Tanner is anyone's guess, I suppose).

Tanner is already old for a Sophomore. If he redshirts this year, he will be an even older Sophomore next year. To the extent that the NFL has a bias against older players (as it would seem), it is not in Tanner's interest to redshirt and then play 3 more years at BYU. If he really does have what it takes to make it to the NFL, he would be better off leaving after the 2017 or maybe the 2018 season. Redshirting this year does nothing to further his goals of making the NFL.

If he is as good as I think he is, I don't think there is much chance that he will still be wearing a BYU uniform in the 2019 season, whether or not he redshirts this year. So you might as well play him now.