The fashion consistency is easy to explain! Much like the Catholic church the Bene Gesserit discovered an aesthetic style that worked for them very early in their existence and then committed to it.
Art can't exist in a vacuum. Any fashion that a costume designer comes up with today is going to be influenced by the fashion of today. The fashion of 10k years from now is inherently unknowable. I've seen lots of ugly outfits on 90s Star Trek and accepted it as a limitation of the visual medium
We only really see the very rich/royalty and members of a religious order though. "common" fashion on planets that don't force you to wear a stillsuit could have changed a ton
The movies are just in a retro phase. You go through a lot of fashion cycles in 10,000 years. For like 2,000 years, everybody wore assless chaps (this is canon).
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100 years from now beehives and bell bottoms may replace FiberOptic Hair™ and Levi's Paint On 601 Yellows®