You can't have it both ways. Either they can get better (ie lower their averages over a significant sample size) or they're can't so they're stuck where they are forever. If they can't lower their averages over a significant sample size then they are stuck where they are forever.
It's one or the other. Clearly, I think we're talking past each other.
Ugh. I think I see where the problem is here.
I am talking about some nebulous number called their "true talent." Which is to say, at any given point, if I were God, I would know their true talent--what their actual ERA was going to be in any given season, based on their sheer pitching talent. We can't measure that directly, though. We try to approximate it based on past in-game results, statistics like FIP and SIERA, etc. And those statistics are
affected by each and every game, and even when regressed our estimations of the true talent are going to differ. But
. That doesn't mean that a single game, or month, or even year in some cases, is going to be enough to convince us that a talent change has indeed taken place. We've been burned too many times before by pitchers that get hot, then cold, then hot, then cold again, or hitters who have one great month and slump the rest of the year. So we require largish samples for the various stats, until we reach some empirically determined point where the latest results actually have predictive value.
I am not
saying that Marquis, Coffey, and H-Rod, and pitchers like them, can't get better. I am
saying that we need a hell of a lot more proof than a single month before we accept that as a likely hypothesis. If you think a month is a "significant sample size," then that is where we will have to disagree, particularly for a reliever whose month often consists of ten innings or less of work. Historically speaking, pitchers over 30 rarely improve without either steroids or developing a knuckleball, and historically the kind of sample sizes you're talking about are way too small to make any long-term judgments about someone. That's what I'm saying. Please don't put words in my mouth and say "so you're saying pitchers can't improve" because that is not what I'm doing at all
. I will say, however, that at this point any changes in true talent are likely to be age-related... and the aging curve for pitchers usually involves a plateau followed by a sudden, sharp drop.