Using that as an example goes completely against what I just said. BYU has had a ton of turnover. QB, the trenches, RB, WR, head coaches and coordinators. Pretty much everything. And not just turnover once in there, but constant. They've had half a dozen different QBs in that time. They've had a TON of different offensive linemen. They've gone from a bunch of experienced senior WRs to inexperience and having to build back up. They've gone through 3 coordinators. What about this suggests there should be consistent improvement?

You should be expected to improve when most of what you have as far as coaches and personnel stays the same over time. When you have one QB for 3 to 4 years. When you have a bunch of OL with 40+ starts. Etc.