That there isnt a lot of upside for TH coming back to BYU.
I see this scenario playing out 1 of 3 ways.
1) Mangum struggles or gets hurt & TH wants to keep playing. TH comes back to BYU.
2) Mangum continues to improve. TH wants to keep playing and because he's a good person he would exit stage left to Michigan (which is the only school I'd see him transfer to) so as to not create a QB controversy.
3) TH decides that at the ripe age of 26 to hang up the cleats.
TH, and rightly so, won't make a decision until he sees how this season plays out.
But if I were him, I don't think I'd come back to BYU. Other than pride (i.e. finishing what you started and being an ambassador of the Church's school), there isnt a lot of upside in returning to BYU.