The population numbers sort of… even out later in the books.

Your right about them not necessarily relying on the spice melange, but they do rely on the juice of the sapho root to accelerate their thoughts and increase their processing speed. So yeah, they’re still on drugs :)

Don’t let the cover art discourage you.

The original series eventually delves more deeply into why the society and technology is in the state we see it during the books, but it’s a gradual shift. Character focus remains the main theme throughout.

You might enjoy some of the “prequel” books written by his son, Brian, and Kevin J. Anderson (allegedly based… perhaps loosely… on Frank’s unpublished notes). They tend to focus more on the setting as a whole. Personally I’m not a fan of the actual writing, but there’s a ton of lore crammed into them.