A case for Miss Marple

I heard this Radiolab podcast a couple of weeks ago. It seems some scientists in Canada have fed the complete works of Agatha Christie into a computer and discovered that in the later years of Christie's life, her work changed fairly significantly. 
Apparently, folks say that Christie's work jumped the shark somewhere in her later years. Later books were mooshy, muddled, and confusing, and not in an intentional way.  This has been known to happen. I'm fairly certain that Kurt Vonnegut's last book was made up entirely from excerpts of his other books (not that I don't love the man). The nice researchers from Canada, however, think that something fishier was afoot.
They discovered that as Christie grew older, her vocabulary shrunk. A lot. Her nouns get squishy - she's using vague or indefinite nouns like "thing." She starts repeating phrases more often. The overall number of words in her vocabulary just keeps dwindling. Anything. Something. That thing.
The Canadians think the mystery of the missing words could have been the onset of dementia. I've read a few articles on the subject now, and there's not a lot of other evidence of such a decline, but there's not evidence there wasn't a decline either. Her last book, perhaps ironically, was called Elephants Can't Remember.
The folks from Canada make pretty convincing arguments. They even ran her numbers against the numbers of writers who are known not to have developed dementia. The vocabulary isn't different from her earlier work, only smaller than, so it's unlikely a ghost writer was involved.
That's what really hit me, actually. Doesn't everybody's vocabulary shrink? My mom's made an art of inventing words in place of words she can't remember - ramistan, freidendorf, hoopdeecak, and others. Sorry, not sure how she'd spell those. She's been doing it all my life, and she seems to have as many of her faculties as I do. My mother-in-law has sound-effects - the self checkout, for instance, is the boop boop. I get paid to find the right word, and right now I end far more sentences with "uh.... lost my train of thought..." or "you know, the thing with the thing" than anyone I've met.
Maybe Christie was just phoning it in. She was like "I'm old. They'll publish whatever I tell them to. I'm Agatha Freaking Christie."
And that's a thing too. I'm pretty sure that Joyce Carol Oates once said that she'd submitted work anonymously, just to see if it'd still get published. It didn't. Tolkien tried to sell The Silmarillion before he got famous, and they told him it was unpublishable - plus I've heard reading it makes you lose your will to live. 
And, you know, Lawnmower Man.