I wonder who all out there really knows the Dune political mythology inside out.

There are certainly enough people who like all of Herbert’s Dune books, but there are lots of people who only like the first one, and even more who only like the first three. So you’re right that the finer details don’t get the attention that Tolkien does.

As for when magazine serials declined, I can only answer the for science fiction specifically. The average SF novel started to get longer in the early 1980s, and SF magazine circulations slowly declined, so those were both factors contributed to the slow decline of SF magazine serials (the Dune novels were unusually long for magazine serials).

Serializing SF novels was still big in the 1970s, then it seriously declined in the 1980s but was still a thing. It looks like after the early/mid ’90s only Analog regularly published serials, and they only serialized a few that were particularly notable after everyone else stopped doing it. In the last few years they stopped doing it all together.

Sorry, but I’m going to have to start talking about SF awards again to answer this question, simply because that’s the easiest way to look up popular novels and were they were published. I should point out that I’m using the lists of award nominees and where they were published that appears on Wikipedia. If a novel was serialized and then later printed as a book, it was eligible twice (stuff was almost never nominated twice, and mabye they eventually made a rule against it, I’m not sure), so there might be serials that were nominated the second time around as books that I’m missing because Wikipedia lists who published it the year it was nominated.

There are usually 5 Hugo nominees a year, so around 100 a decade.

From 1971-1980 16 Hugo nominees are listed on Wikipedia as being serialized, plus two more that I know off the top of my head were serialized in magazines but nominated for their book publication. Every year had at least one nominee from a magazine.

From 1981-2002 there are only 10 total, and none after that. The three that came after the mid-1990s are all by the same person (Robert J. Sawyer).

There was a Nebula winner in 2006 that was published in Analog, Camouflage by Joe Haldeman. That’s The only 21st century Nebula nominee for Best Novel that I see listed as a magazine publication.

Count Zero, from 1986, is probably the last serialized SF novel that you’ve heard of.