Well, there are numerous people who've ignored all the medical advice, been conspicuously idle and self-indulgent, who live to a ripe old age anyway. Or of course done all the right things for decades and dropped dead at 60.
My mum didn't bother with medical advice. She never went to a gym, lived on ready meals and processed food, never took much exercise, and lived till she was 96.
Likewise my maternal grandma got little exercise, smoked and drank, never went to a gym (gyms were a rarity in those days), and lived till she was 91.
Some people say the important thing is not your lifestyle but whether you actively enjoy life and fling yourself into it. That'll keep you going when all the world-weary gym enthusiasts and teetotallers quietly fade away.
In any case, do I really want to live to 96 (or even longer)? I imagine I would be so disenchanted by then (not to mention decrepit) I would be happy to bow out.
But even if you have the healthiest lifestyle in the world, a sudden unexpected illness can come along and disable you - or even finish you off. My sister was having a good life until she developed motor neurone disease at 56 (amazingly, she's bed-ridden but still alive 17 years later). There's nothing she could have done to keep it away - what causes MND is still a mystery.
As for me, I have a fairly healthy lifestyle (minus the gym workouts) but I don't kid myself it'll keep me fighting fit. There are plenty of nasties waiting to pounce.