In 2016 BYU may have had better starting talent than is expected from the 2020 team, but not as much depth as we will have in 2020.
There is a learning curve for every new head coach and BYU hasn't had a great upper class man at QB in 2017-2019. Tanner had a few shinning moments, but didn't develop into a game changer talent. I think BYU is currently in much better shape at QB now that the 3 top QBs have more experience. I believe all 3 have more upside than Mangum.