Hall will get back for 2018. Then Mckee would (hopefully) back for the 2020 season. If BYU signed a non mission QB it would make 5 & 6 scholarship QBs on the roster a couple years which is too many.
QBs in 2018
Mangum
Hoge
Wilstead
Barstowe
Hall
2019
Hoge
Willstead
Barstowe
Hall
2020
Wilstead
Barstowe
Hall
Mckee