I think you can look at Georgia Tech to see an example of "makes a power house football team fall and start becoming ordinary or below average team ?"
From 1902-1963 (61 years) GT Football has an all-time record of 380-191-32 (.630 winning %) and has 5 SEC Championships, 3 Southern Conference Championships, and 5 SIAA Championships and national championships in 1917, 1928, 1952. Georgia Tech's COFH record was 27-26-5 (.630 winning %).
To put this all in perspective, from 1964 to 1982, GT Football compiled an unimpressive record of 104-100-5 (.497 winning %) with 0 titles in the independence span before joining the ACC in 1983. In the ACC starting from 1983 to 2020, GT Football compiled a record of 251-205-4 (.545 winning %) with 3 ACC Championships, and a national championships in 1990.
From 1964 to 2020 (56 years) GT Football has an all-time record of 355-305-4 (.534 winning %) and has 3 ACC Championships, and a national championships in 1990. Georgia Tech's COFH record is 14-41(.254 winning %).
I think we can learn a couple of things from this.
1. Conference Membership is greater than 1 College or University
2. While Coach Dodd and President Edwin Harrison envisioned Tech eventually being the Notre Dame of the South and playing a national football schedule, Georgia Tech never reached the glory days of their time in the SEC and Georgia Tech joined the ACC in 1983 as they needed conference membership to survive.
3. Coach Bobby Dodd and President Edwin Harrison took a power house football program and jump started it in becoming an ordinary or below average program