Yes, there are people who eat all meat or all “animal-based” diets and report feeling quite good and that they have plenty of energy.

I don’t think this type of diet is optimal for everyone, especially over a long period of time because there are benefits to consuming things like polyphenols that are found at high concentrations in many plants. Also, fiber can be a beneficial nutrient, but it does depend on the type of bacteria that are present in your gut. Also, one of the great things about being a human is that we can consume so many different types of foods and create great memories of eating with family and friends…consuming a very restrictive carnivore diet is going to limit the types of experiences with food that we can have.

That all being said, nutrition is very personalized. I wouldn't try and convince someone not to eat only meat if they were enjoying it and feeling great.