Dude, when the "backup" is better than the starter, and continues to play when the "starter" can play...
He's no longer the "backup". Period.
If BYU wanted to play Mangum, vs. Utah, they COULD have. Mangum played against New Mexico State (before Utah) and Western Michigan (after). They played Wilson... because he was better.
When Joe Montana went down, and Steve Young took over... the 49ers decided to keep Young, and sent Montana to KC. (Tanner is no Montana, and it's exceedingly unlikely that Wilson turns out to be Steve Young.) If Montana was still the better QB, the 49ers would have kept him, and traded Young.
I'll concede my comparison is not totally analogous- Montana was out for the season. Mangum wasn't.
Wilson replacing Mangum- better player.
Shelley replacing Huntley- "worser" player.