Five reasons writers don't improve with age

I had dinner with my old friend Bob Klonowski last week, and he asked me an exasperated question:  why do writers seem to get worse as they get older? If writing is a skill (and it certainly is), why shouldn't increased experience with technique, as well as extended encounters with other human beings, cause writers to improve? He had had several bad experiences with writers he had once enjoyed, and was interested in an explanation.

I didn't have an answer, or even any good counterexamples, but in a related article, Terry Teachout, recently wrote about artists who stop producing, indicating that this is often a good thing, because later work, when produced, tends not to match the quality of earlier, thus bolstering Bob's point.

"I'm a big fan of his early work" is a standard joke.  But is it true?  And if it is, why is it true?

I had a couple of explanations for Bob, and a couple I thought of later.

1. Lack of editing.  Many writers are more the creations of their editors than they would like to acknowledge.  Chop the worst-quality 10 percent (or 20 percent, or 30) of any work, and it likely would be improved.  In a writer's early days, the editorial requirements often cause that to happen. When a writer starts selling, and has a reputation, editing falls by the wayside. Even formerly perceptive friends and colleagues are deluded by the simple fact of success into thinking that editing is no longer as important.
2. Related to lack of editing is the simple fact that a writer has to be better to make a reputation than to keep one. Once a reputation is achieved, reviewers look less carefully, and readers who have developed a taste for what the writer has to offer are willing to put up with a diluted or cut product in order to get it. Marketers have an existing hook to sell the books with, which makes sales relatively easier to achieve.  Having market dominance means that quality is less important as part of the total sales package.  And, since quality is painfully hard to achieve, and successful writers have speaking gigs, tours, and other distractions to contend with, they are disinclined to torture themselves.
3. With increasing age, there are basic failures of cognition. Sad, and agist, but unfortunately true. The number of words you can see ahead goes down. The skein of detailed relationships is harder to perceive. The reader of the final book doesn't see it, but the act of writing is as fraught as surfing and as reflexive as Whac-A-Mole. It gets harder to manage, and thus scarier, leading to writer's block. One hopes that increasing experience can compensate, but sometimes it doesn't.
4. Some writers just don't have that much to say. They have a few experiences, some key perceptions, a limited repertory company of characters, and a couple of verbal tricks. Book One is fabulous and original, Book Two revisits the successes of the first, Book Three is a chastening attempt to try something new that doesn't come off, and successive volumes after that are rewrites of Books One and Two.
5. And, finally (related to the above), maybe the original books weren't really that good in the first place. You were the right age and having the right experiences to be charged up when you read them, but if you read them now, you would like them no better than that obese volume squatting loathsomely on your nightstand that some enthusiastic review persuaded you to buy against your better judgment.

Not all these explanations apply to every writer, of course.  Maybe the trick is to start late in life, like Penelope Fitzgerald, after some other career, thus avoiding unfortunate comparisons with youth.

Anyone know a decent writer who became better with age?