Another theory I have is technology and it is impacting everything. Training and nutrition improvements across college football leading to more parity. If you look at the training, recovery and film rooms across major college programs you see the same thing you see at the NFL level. The big high school programs have training centers that were are on par with colleges 25 years ago. When everyone gets the same access starting at 14-15 years old it closes the gap from great to mediocre.
We also haven't seen a true freak at WR come out in a long time. We had guys like Moss, Megatron, Julio Jones, etc coming out every 4-5 years for a while there. We lack those big, fast WRs that break defenses.
Of course the RPO and spread offense variants has changed the type of QB coming out of college. I was thinking about this the other day. I don't know if a guy like Brees even gets a scholarship offer to major school. His size alone breaks analytics and scouting programs are now relying heavy on size/weight/athletic ratios at every position. These systems are also wanting speed everywhere. Now those 6'4+ freak wideouts are being turned into TE's in high school and college.
Finally analytics is playing a huge roll. Every team is chasing the same goal and scheme with defenses trying to rapidly catch up. This means players need to be versatile.
I think we'll continue to see skill players get leaner and faster while linemen stay the same size. this happened in basketball much faster. Analytics changed the entire way the NBA is played and rosters are built and did so really fast.