SPOILERS BELOW: It think this is an oversimplification. Mentats still had agency--both Piter de Vries and Thufir Hawat have their mentat schemes to kill the B

SPOILERS BELOW: It think this is an oversimplification. Mentats still had agency--both Piter de Vries and Thufir Hawat have their mentat schemes to kill the Baron Harkonnen. One of the big points of the Dune series is the indeterminacy of the universe. Computers, being deterministic, couldn't think properly to handle the full universe, so humanity's subjugation to computers (i.e., "machine thinking", both as rule by computer and as habits of thought for humans) was a dead end. Dependency on computers also meant people didn't develop human skills. After the Butlerian Jihad, the Guild, the Mentats, and the Bene Gesserit all became schools of human development. Mentats aren't just human computers who perform calculations quickly. They cultivate almost zen-like mental states that allow them to sift patterns of data into conclusions, among other techniques. The later books go into much more detail, but part of the point is that they're not deterministic thinkers; they're practitioners of very advanced modes of consciousness.