Again, these are not mutually exclusive. Even if the idea of a keto diet has been around for 100 years, that doesn't mean that right now it is not a fad (a style, activity, or interest that is very popular for a short period of time). It was never popular before, it is now and it almost certainly won't be in 10 years or so.

>Are there many diets that have lifelong adherents over their lifetimes over the last 100 years from the general population that have been studied?

who are these people who have done keto their whole lives and have been studied? Can you share the case studies?

>Keto is a fad >no it’s a diet developed in clinical setting been around more than 100 years >I don’t know why people bring up that its not relevant >yes, it directly contradicts what OP said >it’s no mutually exclusive, sure it’s been around 100 years, but it’s a far and won’t be around in 10

>Are there diets that have life long adherents over the last 100 years that have been studied? >show your studies

I think someone like you would be better off googling. You should easily find studies supporting as many as 70% of epilepsy patients who are treated with keto as children can get off of keto after 5 years without any seizures and no longer need any medication. You should also be able to find studies of long term keto diets, ‘long term’ as defined within the studies themselves, covering various types of individuals and get yourself a healthy perspective on the overwhelmingly positive outcomes and to a fat lesser extent the negative outcomes.

Good luck!