That wasn't a problem at all. Not 100% ideal, but it worked pretty well with how they converted guys.
But the two years after is when they had a bunch of guys from Bronco's bad classes, plus youth and then added injuries. At that point, there's no reason to flip flop. Stick with it and develop guys. 2017 had to deal with the offensive woes killing the D, and 2018 was actually a top 30ish D.
You make it sound like things completely fell apart and it was horrible. That happened to USU when Gary first went back and they tried flipping to a 4-3, but that's not what happened to BYU. They were fine so let's not exaggerate.