"don't you think that you can write many good books? i mean, fitzgerald's novels aren't better just because he wrote so few, no? i think the basic assumption of "lots of output = bad output" is a flawed one."
They're not better because he only wrote a few, but they also don't suffer for quality because, for the most part, he was able to take his time with them (this is true of some more than others) and didn't need to just crank them out one after another, or make The Great Gatsby 7 because he was contracted for a book series.
Lots of output doesn't necessitate bad output...to a point...
But at the same time, I'd argue there comes a point where the urge/mandate to churn out books and sequels one after another causes the works to suffer in quality, it becomes more a manufacturing plant than individual craftsmanship.
"take asimov for example, or simenon, or dumas, or christie. all of those have a huge bibliography, yet all of them are high-class writers (even if i don't like christie that much)."
I'll use Christie for my point here, as I think she fits well--
The thing with Christie is that there's something of a pre-set formula to the detective story. You can fiddle with that formula (and she does) but still, having that formula pre-set for you allows you to churn out works faster.
When you KNOW the book is going to open with Conan Doyle's Watson narrating to the audience or will star Sherlock Holmes or Agatha Christie's Poirot or whomever else...and when you know a crime will have been committed, the heroes have to unravel and find clues to said crime, there are obstacles to doing that, likely red herrings, likely iterations of different detective novel stock characters...
All of that, I'd argue, lets you churn out a novel and novel series faster than starting from scratch and, formula free, setting down a comparatively-original novel like "This Side of Paradise" or "The Great Gatsby," where the plot, story, characters and everything else have to be made from scratch...
There's a "mold" of sorts already in place for detective fiction and a lot (not all) sci-fi and fantasy fiction...for as fun a read as they can be, there's a definite mold and formula to the Harry Potter and Hitchhiker's Guide books...
There is no "Great Gatsby mold" or "Pride and Prejudice mold." They benefit from being unique creations that way.
Using some mold or formula to shape your book's structure is inevitable--ie, having chapters, first or third-person narration, a three, five, or two-act structure--but even for those like Christie who wrote good works with a formula, I'd rather have the works that are mold-free and almost always works that are one-offs or, if they must be a series, a PLANNED series (there's a big difference between Tolkien planning a sprawling trilogy and some author going "Ummmmmm...and then this happened, I guess...and then this..." to try and milk a 5th sequel from a book where he only planned to write one novel but is now trying to pump that cash cow for all it's worth. In fairness, hey, gotta make a living...but that doesn't mean that's good art.)