I don't know that you can really draw a line between "good" and "hack." Most of the time it's a gradation, and even when it's cut-and-dried.... some pot-boilers and formula stuff is great. And some very original literature is lousy.
In the context you brought up - the sub-category of an author or series going downhill - I don't think we can generalize. I know authors who took ten books to really get great. I know others who go up and down from book to book. I've seen series where the first three books were okay, then a string of great books for three or four, then downhill.... and then around book twenty, the author "reboots" and gives us something great again.
I think, on a philosophical level, it would be great if authors could mix it up more and always stay at the top of their game, but the realities of the business just don't allow for that. In the old days, someone like Agatha Christie probably had the most power to do what she liked because she had so many popular characters and she was so prolific that she could jump from one to another. Publishers often want to rein in prolific authors and keep them on a managed and marketable path in terms of their book releases.
This is one of the reasons, btw, that you got people hyped about "pulp fiction." In my opinion, writing pulp is what kept so many writers really fresh in the old days. You write for the story and the readers, not for your reputation. Writers who wrote for the pulps developed so much skill and control, they could do almost anything when they applied it to their "real" writing.
And after all that, I do have a measure of quality. It's a rule Lawrence Watt-Evans quotes as the only rule of writing: "Thou Shalt Not Bore the Reader."
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