Sometimes you DO lose your starting job due to injury. Sometimes even all-time greats and borderline Hall of Famer's lose their job due to injury.
Tom Brady took over for an injured Drew Bledsoe and the Patriots never looked back. Bledsoe is Top 10 all-time in yards and completions and Top 20 in touchdowns.
Steve Young took over for an injured Joe Montana and the 49ers never looked back. Montana is considered one of the best all-time
Sometimes losing your starting job isn't an reflection on your skills, talent or potential. It just is time for the team to move on.
At his current rates, Tanner Mangum is going to finish this season with over 3,000 yards passing (and will likely be pretty close to 3,500 yards passing). He is likely going to finish the season with close to 25 touchdown passes. He will likely complete more than 300 passes.
Taysom has been a pretty good quarterback for BYU. He's as dynamic an athlete as BYU has ever had at any position. But Mangum is going to be the entrenched starter following this season. And he is only going to get better.
If Taysom wants to play again for BYU, and if I were the coach, I would move him to another position to get him on the field. But my QB going forward is Mangum.
I wouldn't trade three years of Mangum for one year of Taysom.