Tanner's comfort level with being a backup again makes no attempt to explain why Taysom would come back.

Possible reasons why Taysom might play at BYU in 2016 might include:
-perhaps Taysom is loyal to BYU and can't imagine himself playing anywhere else;
-perhaps there is a graduate program at BYU he wants to experience;
-perhaps he has family at BYU or in Provo that will keep him there, as in maybe his wife is in a program and needs to be in Provo for the fall 2016 semester in order to graduate;
-perhaps he has a solid bond with Bronco or Anae or the team;
-perhaps he had a vision and feels that he is called to come back as it is the greatest form of missionary work he could do;
-maybe President Monson has called him to the BYU QB mission.

To be clear, I have no such intel on any of the above and some of the above I can't even necessarily fathom happening. But Tanner being comfortable as a back up is not a reason for Taysom to stay.