> people will live to 125 quite steadily

Only after the current generation(s) of doctor(s) dies. And only if you make this in pill-form. Otherwise people will be people and won't even go to the gym.

It might be also the reverse, they develop a powerful+personalized drug that brings heaven on earth to your neurons (first time heroin experience + sexual gratification + childhood fulfillment + extremely addicting etc etc etc).

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Now that I think of it I'm gonna go with the latter.