“The books went downhill rapidly:
Dune
Dune Meshuggenah”
In the old science fiction discussion groups they called it “brain rot”.
The first book of a SF writer was often their best—it was the one that got them their reputation.
Most authors could write one or two more good books before the brain rot set in...
There were a few exceptions but as you say Herbert was not one of them.
Asimov had a good run but the later stuff became unreadable. The last story that didnt bore me to death was The Gods Themselves.