You do know that even "Veteran big leaguers" can and often do show wild variances in performance from one year to the other right?
Cal's a great example since he had so many seasons at 161-162 games.
His rWAR dropped by 7.4 between 1991-1992. If you want to say it was because he was over 30 he dropped 10.0 to 5.6 from 1984-1985.
Lots of guys just have a random down year or career years that they never get close to again.