Their talent is measured by a single standard, which is constantly declining. That's the trick. Unfortunately, any significant statistical measure of pitching skill is polluted heavily by context and sheer luck - saves, wins, Losses, earned runs, hits allowed, homeruns, strikeouts and walks. All of these results are the product of the pitcher's skill, managerial tendencies, team offense, team defense, skill of the opposing pitchers, skill of the opposing defense and offense, visibility, park effects, weather effects and umpiring. It's just too many variables.
Chaos theory sets in.
And I didn't mention health. The randomizing effect of chance can take a very long time to average out. So long, in fact, that basic skills can significantly change before they are recognized at all. I guess that leaves voodoo and tea leaves as our main analysis tools. Well, you might wonder what my point is.