There's a lot we don't know about nutrition. It's kind of amazing how much we are guessing about. But there are studies, and dietary fiber has been a major focus of nutritional studies for a couple decades. There is a lot of statistically significant evidence that increasing DF is good vs a standard diet. Nobody is studying it versus a carnivore diet that I've seen.

However, we can definitely zoom in on one area. Cancer. There are some pretty good reasons to think carnivore diets increase risk of colorectal cancer while fiber decreases that risk. And fiber's role in hormone regulation isn't nailed down but appears to have good effects on some cancers too. But that's just one set of risk factors and there are a bunch of others to consider.

Anyway, the point of all this is not to say that a carnivore diet is bad and fiber is good. It's just to try to ground the conversation in some of the actual literature (e.g. https://www.sciencedirect.com/science/article/pii/S2213453021000677) and to start trying to identify tradeoffs. Like, if you're on a carnivore diet, please do your annual colon cancer screening. Do your blood panels religiously. Maybe keep tabs on your gut biome. Read the studies as they come out. You're way out in the fringes and you'll have to take responsibility and control. Managing your health on vibes and intuition is not the way to go.

As for myself, I spent a decade working on my dietary issues and reached a pretty good point: low carb but usually not keto. I augment with fiber (chia seeds and whatnot) when I can. I feel good and my blood tests get thumbs up from my doc.

Every body is different. Good luck figuring out what works for you!