We don't know the long term impact of ketosis on the human body. Anyone doing it is experimenting. This is unlike what we know about a diet primarily of plants.
We do know the negative long term impact of people claiming they are on a keto diet but not in ketosis.
1) Most Vegetarians I knew were fat and unhealthy, while claiming to be healthy (SDA's, I looking at you!). Most Vegans I know look like they are refugees from Ethiopia. Sure, you might be able to provide a fit, reasonably muscled vegan, or a relatively thin vegetarian, but that can be hardly be credited to their diet.
2) If someone is doing a keto diet and not in keto, then by definition, they are not doing a keto diet, or they have something medically wrong with them.
I am not pro keto, or anti-keto. But there were (and are) people groups who have lived for centuries with 95% of their energy intake coming from fatty meats. Some of you have such low respect or evolution... You think there is only one fuel source. New Flash: The body is a complex, very complex organism that can survive (and thrive) on many different diets. Some of you pretend to know more than you do, and others pretend like a lack of evidence is evidence. Go ahead and "wait" for science to tell you something that is already quite intuitive, if it ever does.
Human physiology is so complex and we still don't know as much as we think about it. We know things empirically, but not why or the inner workings of it, which makes many recommendations merely correlation and not causation. Many things are credited to the "reason" and yet, they are often proven wrong as science advances.
The arrogance I find nauseating in this thread is that there is a "special" and "healthy" diet, like some sort of "key" to the "fountain of youth". There is no key and our adaptable body has many ways to survive and thrive. With the way some of you think here, it would be good to remove gravity, so that we don't have to work so hard to move, stand etc... Until you realize, wait... That is what keeps us strong and healthy! Without it, you fall apart, literally. Just like people who always wash their hands so as to remove any source of something "bad" and yet they can't build any immunity because of it.
The body is powerful people and it has a time limit. You will die. Spending your precious time wondering if a carrot stick or a beef stick is going to give you an extra 5 years on this earth is... wasted time. Just like my post here is. This should be quite intuitive. Were not talking about a diet of drinking draino and anti-freeze, so let's stop the hysteria.