I never said or even meant to imply that the 25% turnover figure meant "average or above starters". That is obviously absurd (not to mention a slippery definition).
Not doing the research on averages, but taking the Buffalo Bills. Dick Jauron changed 10-13 starters (I'll pass on the average or better angle) from his 08 to 09 teams depending on how you want to count. And that wasn't even a "rebuild" situation with a new front office and coaching staff.
Another example: Seattle has only 3 starters left from before Pete Carroll took over, at the same time Buddy Nix came to Buffalo.
Now, the league-wide average is less. Good teams that win have much less reason or inclination to wholesale change out all of their starters in this fashion. The good teams, as one would expect, have the least amount of overall turnover in quality starters all the way down to 3rd stringers. Every team makes changes.