RB
Squally, when healthy, was very good. In '17 and '18, he averaged 6.2 ypc in games where he had more than 10 carries.
Don't forget either, that Squally was only the second best young RB Bronco left. Francis Bernard was the favorite to be the starter over Squally after Jamaal graduated, but Kalani moved him to LB.
QB
As for QB, Mangum Threw for 3,377 yards and 23 TDs In 2015. He was a Freshman all-American with a years worth of starts under his belt. This is what Kalani received. However, what saw the field in 2017 and 2018 was not the same QB we saw in 2015. Why the drop in play?
Mental health challenges and coaching. I don't know how much effect to assign to each factor, but both contributed.
Summary
The QBs and RBs Bronco had lined up for the 2017 and 2018 seasons were very talented and projected to be great players. However, due to coaching decisions as well as things events outside the coaches control, they never reached their offensive potential under Sitake's staff.