I absolutely agree with the OP's point, if you didn't have to develop rookies then why have position coaches, learning football takes an entire career.  I think this where the 'prime years' term comes from, keeping your physical abilities while learning enough about your position to play at a fantastic level.

 

Though I do think that regressing can throw a wrench in the development scheme: Did you have one lucky good year, or was it one unlucky bad year.  I think Blake will have to prove that point.  In the development curve you'd like each future year better than the last.

---