If you're being realistic about what BYU is, and consider what the previous staff had as their system and the players they left, it adds up.

1) Bronco obviously didn't leave much up front. They already ran a 30 front, and not many guys for the future could easily transition up. In 2016, luckily guys like Tautu made the transition well. But there was nobody for the future. They tried with Takitaki for a year out of necessity, and it worked ok, but wasn't ideal.

2) Again with nobody for the future. 2017 is where Bronco's crappy classes in 13 and 15 start to hurt on both sides of the ball. Cupboard is empty.

3) BYU has a lot of missionaries, obviously. So for a normal program with non mission kids, Sitake could have a bunch of guys from the 2016 and 17 classes start to get in right away. But this is BYU, so some of those recruits are on missions. Think about this. Normally, Tyler Batty is a freshman working for PT in 2017. Instead he's a missionary and just now a freshman.

So it's not like they didn't recruit guys. It's a combination of missions and injuries.

Tuifua is a star DE recruit and he has a career ending injury.
Tofa battles injuries up until 2020.
Leiataua and Dawe, some of the few leftovers from Bronco's classes, both spend most of their careers with injuries.
Pilimai mission
Fevaleaki mission
Baker mission

All of these early guys on missions. Keep going later. Albright, Daley, Schoonover. Too many missionaries. Too many injuries.

It'd not that they haven't been recruiting guys. It's a combination of taking forever at BYU, and seeing the implications of that when others get hurt. Tuifua should be a star senior right now.

It didn't take long to find them. It took long to get them into the program and staying healthy.