fiber is great and under-consumed (especially soluble) and is very well-studied on modern populations [0]. we don't really need to make mechanistic reasonings about evolutionary adaptations that occurred hundreds of thousands of years ago or rely on studies of a small, homogenized population whose lived realities are far from yours and mine
plus, following the the hierarchy of evidence, mechanistic guesses are considered extremely low quality, only ranking above things like mouse studies [1]
[0] https://www.uptodate.com/contents/healthy-diet-in-adults#H61... [1] https://en.wikipedia.org/wiki/Hierarchy_of_evidence