This isn't the first time BYU has been in this situation.
For all you whippersnappers in CB, the current situation is pretty close to the Wilson - McMahon saga of 78 - 80, with 2015 being '78.
Wilson, went down with a season ending injury in 78' midway through the season, McMahon comes in and actually looks like he is a better QB as a Soph and finishes out the year going 4-2 vs 5-2 for Wilson, but Wilson had the tougher schedule.
McMahon then sat out the entire 79' season, Wilson was an AA, and McMahon improved and redshirted.
McMahon then had monster 80' & 81' seasons, sitting the year didn't affect his progression or his draft spot, in fact he was a much better QB in '80 than he showed in '78.
That is what you call the model situation, I can't imagine why any fan of BYU wouldn't want to try to repeat that scenario, and would actually wish for Taysom to go somewhere else (looking at you Criddler) to finish his college eligibility. If you want what is best for BYU in terms of wins over the next several seasons, that is what you should be hoping for, and I think the comparison of Mangum to McMahon as far a QB impact at BYU goes, is probably my best guess at the moment.
It drives me kind of crazy that we have the 21st century equivalent of Steve Young and Jim McMahon on the team, and our own fans and former players are actually advocating for one of them to transfer to make room for the other...mind blowing to me.