Agree with most but have a few other ideas as to why the game has changed, The percentage of bad, average, good, and great coaches has remained the same IMHO. We had some terrible coaches in the 80s, 90s, 2000s, 2010s just like now. Just a few reasons to how we got to the current game.
1. Single wing/shotgun. The single wing was one of the most effective offenses in football. It has morphed into the RPO/Shotgun offenses we see today, The gun eliminated the full back position, and 21/32 personnel. Its just more effective and has more attack angles than traditional under center offenses. This has changed how defenses, passing games, and run games operate. These new offenses are more adaptable.
2. YOUTH FOOTBALL. Youth start playing football too young and there are too many youth teams and not enough players. 9 years old was the starting age in the past. Now there are 5-8 year old tackle teams with single group age limits modeling basketball. You sometimes have a hard time fielding 15 players for an age group. So you have bad practices, switching player positions over n over, and teams merging just to play bad football on a Saturday. Then you have super teams running through leagues creating mismatches. With CTE awareness going up most of your top athletes are playing basketball, baseball, and lacrosse. Most of your top HS players now either come from top youth programs, or top middle school programs that hoard talent. So these weaker football players sometimes end up in HS lost and confused. The average joe football player is not the same avg, joe from years ago mentally. Youth football has some of the worse coaching and not preparing players for high school.
3. Agree with coach about the backpack coaches. Think the quality of assistant coaches has gone down coaching wise. The most successful HS have a great staff, not just a great HC. An assistant should be striving to be a HC. Studying the game, learning game theory, and learning how to communicate with todays youth. The reason these backpack coaches get jobs is because they can communicate with parents, players, and know how to recruit players to your school. If you want to succeed you need 2-3 "backpack coaches". They know the kids, their situations. They know the lingo. You can be great X/O all you want but if you cant edit an Instagram video, pick johnny up for practice or counsel johnny when the opps are after him you not effective. So the staff needs these coaches in minimum quantity and not coaching major position groups.
Lastly like I mentioned about youth football. There are just too many high schools schools and not enough quality players. Then the top talent hoards at certain schools. When I worked USA football that was our biggest issue. Not enough kids playing due to safety concerns, We have to find a way to play football safely and build the numbers back up, and the quality will get better. IE Charlotte had 13 public football HS in 2000, and in 2025 its 21. Then add in charters, private, and Parochial. Just not enough players for everyone to be decent,