I have that same one word: moderation.
Not just moderation in fiber intake, but in anything.
After you establish a moderate and consistent diet, your gut will make the necessary microbial adjustments to handle what you eat.
If you want a paleolithic rationale for eating some fiber, imagine what a paleo guy would do if he found a tasty leaf or flower or root or fruit. And if this p.g. lived where leaves and flowers and roots and fruits are abundant, he would eat those fibrous foods without counting soluble and insoluble. We are still that way: I remember as a child knowing every good tasting grass and weed on my walk to school, and I learned the good ones on my own. So, I think it's in our nature to nibble on plants, and that means some fiber can go down the hatch.
I'm looking for rationale not to eat fiber. It seems like it's very damaging to the digestive system...
I feel like most fruits/veggies contain most of the insoluble fiber in the skin, which we would have removed like many of us still due today. Fruits are easy to breakdown by chewing, and are almost predigested if properly vine/sun ripened. I feel like the breakdown of many vegetables and tubers is not so easy. I've also read that the Hadza way to eat tubers was to roast and remove skins... If eaten raw they chewed on them, and spit out fiber(same concept as juicing).
In today's world we have no reason to consume a food that is harmful to us in anyway. If we can avoid fiber and still get all the nutrients we need for health, strength, and longevity from animal products, why wouldn't we.