but it has been really happening since the late 80's.

Here is the cause of BYU's football demise:

1.) Teams figuring out how to defend the forward pass. With the loss of system advantage, and with current technology and movement within the coaching profession there are not system advantages that exist for more than a year, BYU didn't have the athletes to beat good teams- they never did.

2.) Mid range programs deciding to put resources into football/85 scholarship limit. Now the Colorado States and Utahs and Boise States could start to get enough good players that BYU could no longer win 9 games a year by just showing up. The bottom and middle of college football started to improve significantly in the late 80's and early 90's.

3.) Changing LDS Culture and lack of growth of BYU enrollment. In the 90's Church leaders even would go so far to encourage kids to attend local public universities and attend the growing institutes. Not just changing the cultural pull that kids in the 60's-80's felt to go to BYU, especially LDS kids not from Eastern Idaho or Utah, but starting a trend within both Church culture and membership that spending resources at BYU was "unfair" to the kids who cannot or will not attend BYU.

4.) Rise of Utah. This coincides with #2 but the Pac jump was a pretty daunting challenge for BYU to overcome. Losing to Utah should be the norm but it makes the fanbase batcrap crazy and the envy clouds judgement and decision making.

5.) The growth of money in the sport of college football and how unseemly BYU decision makers find it. I don't believe they care about competing on that stage because I think they believe it passed into immoral a while ago.

Take courage, trends can change and to be honest Lavell probably never should have happened in the first place. Things could change. I have no idea how but likely I could not have predicted Lavell in 1972- but that is when he became the HC of BYU football and when I was born.