Schools didn't abandon the West Coast offense and gravitate toward spread offenses on a whim. They did it for a reason. Or several reasons. But the foremost reason is press coverage. It doesn't just work against BYU receivers. It works great in the SEC, too, which is why you don't see much West Coast in the SEC and ACC, where receivers are much faster than what we see at BYU.
Detmer was raised on the West Coast offense and that is what he ran at BYU and for most of his career in the NFL, where the speed of the game is much, much faster. I think he has seen ways to adapt to a faster and more physical game than what he played under. One of the adaptations is to run the QB more.
I think it is pretty silly to assume that Detmer will run the same offense that BYU ran in 1989-91 because that's when he played. The game has gotten faster and the amount of jamming, holding and pass interference allowed by officials has gone way up.
Detmer will run the offense that works. I wouldn't be shocked to see an occasional QB sprint sweep and an occasional QB draw under Detmer. Those plays punish a defense that is overly aggressive in getting locked up with receivers.