But low fiber doesnt _cause_ those diseases in the same sense as low vitamin D causes rickets. Most fiber-related studies I've seen were small and published in obscure journals. I'd agree the average person (with an average diet and lifestyle) probably benefits from getting enough fibre. But that's in conjunction with other factors. I just disagree with the statement humans "need" fiber.
Your gut cannot be healthy without fiber. It's important for the digestive process. There's a reason dietary guidelines include fiber, and I'm not sure where you're reading these journals but your agreement/disagreement is irrelevant to the point. A lack of fiber does _cause_ blood sugar fluctuations, cholesterol increases, constipation, etc.
Just because it's not as severe as rickets or scurvy doesn't mean people should ignore the needs of their gut microflora, especially given that the majority of cells in our bodies are (mostly beneficial) bacteria.
Eating less sugar and replacing it with nuts and seeds, and not eating starchy vegetables and replacing them with leafy greens will increase your fiber consumption like 4x or 5x. I seriously doubt someone doing like bacon butter and steak keto will claim improvements in their digestive issues unless they were like very sensitive to an allergen or something and didn't know it.