metalprof
Ken Luther
Hmm, I'll toss out an exception, IMHO.
I would argue that Michael Moorcock's literary output has gotten much better over time, even as the frequency of book releases has decreased (so he has more time to craft his newer novels).
My experience with Moorcock is limited to the Eternal Champion books as packaged in the 15 Dark Fortress collections. ("Limited" is a strange phrase to use with respect to that many novels, though!) I thought the early books were great (early Elric, Hawkmoon, Corum, Kane of Mars). But I thought the later works were more hit or miss. And the books packaged now as "The Elric Saga: Part IV" by the SF Book Club didn't hold a candle to the original Elric novels.
So in my perception (and in my occupational language), the slope of his quality graph did end up going negative But perhaps not as negative as others!
I can't speak to Storm Constantine since I'm still waiting to follow up on your recommendations and read her stuff.
Ken