More anecdotal evidence, but while my wife was a huge HP fan, and read all of them, I also stopped at Goblet of Fire, for much the same reasons @Tzigone and others pointed out: bloated, added bunches of new characters to expand the story, and too many damn plot holes / internal inconsistencies.
But, in part, that was because I was burned by Robert Jordan, who (IMHO) did the same thing in book 4/5 of Wheel of Time. Making too much money/fame (IMHO again) to end it where they probably originally intended too, so introduce tons of NEW story and character and stretch it out.
Where it eventually became a bloated mess where I didn’t care about 80-90% of the characters anymore.
I can’t imagine trying to cram the Nantucket or DtF trilogies into single novels. I can’t remember if I gave up during the fourth or fifth Change book, but I’d love to have another Nantucket story.
The way I see it, there are several ways that authors “can’t keep up”…
One is where there is supposed or expected to be a Grand Arc, but in trying to live up to that intent/expectation in a truly Grand way, you write yourself into a corner or into multiple mutually nonresolveable character/subplot corners. So to distract from that you go “squirrel!!!” and wander off into yet more tangents including collateral subprojects, “lore”, “labor of love” pieces, etc.
A specific variant of this is when you did not know at the start if there would be enough traction to justify a series so you front-load it into the first book or first sequel and then it does sell and contracts are signed for more and you have to stretch the remaining bits thin to fill the arc and/or begin throwing in extraneous things to pad it. That last bit puts you again at risk of wandering out onto a dead-end or recursive loop of your own making with no good way to get back to where you were going.
That often ties in with the aforementioned phenomenon with it becoming a superseller franchise, where the author becomes Too Big To Edit.
On yet another facet are those series that are no longer (if they ever were) big top charters and by now the author is blatantly just phoning it in, or in mental decline, or just DGAF any more and has let their freak flag fly … BUT are proven reliable enough bill-payers that the agents, editors and publishers just let them keep going as long as nobody is getting arrested, and they’ll stop publishing when they stop selling.
Or when the author is full-on dead, as with Tom Clancy. There’s a new Jack Ryan and a new Jack Ryan Jr novel coming out within a few months. I think his estate has now published more novels than he ever did when he was alive.
that happened with vc andrews she died but they had ghostwriters putting books out in her name to dodge the taxes owed on a technicality but there were so many that they made more money than she originally did and the IRS noticed that a dead lady was making money and after an audit they had to admit what they were doing and pay back income taxes on it
a lot of her fans were shocked although they did note a change in tone