"There was no basic trade going on"
I'm not sure what logic you are using to think this is wrong.
How much stuff do you think we would import from Mars were we to establish a colony on it?
Even one that was hugely successful and producing a surplus?
The only thing that would justify space shipping would be super high value, low weight items like "spice".
Elon Musk could afford to have stuff shipped back from Mars.
Normal people couldn't.
The fact the whole economic basis of space travel in dune is super rich people prolonging their lives seems reasonable to me.
Current trends on earth mean space travel is now in the hands of wealthy individuals, not just governments.
Sci-fi settings with "space truckers" moving around holds full of space wheat are typically the unrealistic ones.