A first year OC isn't going to take a step back. The OL under Empey... The OL that really only lost their most inconsistent player. The TE group. A QB who got to spend time learning something that most QBs need to (and will in the future) spend time learning before they jump right in. A QB who was already a much better passer than the previous one.
The RB group? No Jamaal but it's not like they don't have anyone. Other than that, I don't see a step back anywhere.
Don't take my word for it. Detmer has said over and over, and it's been evident through recruiting, that he wants to get back to TEs and RBs playing a huge role in the passing game.
Truth is, years like 2015 are an outlier with receivers. It's never that deep at BYU. Look at the big offenses when BYU had guys passing for 3500-4000 yds. Who were they passing to?... TEs and RBs were huge.
Unless BYU manages to lose 3 starters on the OL again, I don't see where there is really room to expect anything but improvement.