Herbert's Ecology Is the Real Subject
Strip away the politics for a minute and what remains is a book about a living planet being consumed. Herbert was explicit about the warning he was writing, and it is not hidden under the sand. A world with one export, extracted by an industry that poisons the ground it works, and an indigenous ecology that fights back.
The worm is the centre of that argument. It is not a monster in the ordinary sense and it is not really even an animal; it is a mechanism, a scale, a geological event that moves through the sand and eats what crosses it. Everything the planet does to humans goes through the worm. The shields exist because of it, the walking is a discipline because of it, and the riders' relationship with it is closer to negotiation than combat. The predator is enormous, indifferent and part of the system, and the humans who live there have arranged themselves around it rather than pretending it can be killed.
The sand itself is the second half of the ecology. It moves, it buries, it silences, it takes moisture out of the air and hands it back only to a machine that costs more than most houses are worth. The desert is not an empty region waiting for heroes. It is a working landscape with rules, and the rules kill the careless.
The water discipline completes the picture. Every drop is tracked, every delivery is a negotiation, every wet garment is a scandal. Herbert uses that arithmetic to make an argument about scarcity that is much sharper than the plot requires: when one substance can be sold everywhere and another can only be obtained locally, the people who control the local one have more power than the people who own the faraway one.
Which is why I think ecology is the real subject. Remove the ecology and the first novel becomes a story about a coup. Restore it and the coup becomes an argument about what happens when an extractive economy is the only thing a civilisation knows how to do, and the extraction is beginning to fail. The books keep coming back to that, in every generation, under different costumes.