Nobody "just dies". Most "natural deaths" are still heart failure. The rest are other kinds of organ failure.

For a lot of people that die natural deaths they could have lived longer if they had kept fit. It just gets really hard to either do or justify when everything hurts and gets harder to do year over year. Or when you are confined to a wheelchair since your early 20s, were told you would only live two years, and then beat that prognosis 27 times over.

But if you are able, even small amounts of cardio exercise could dramatically extend your health and lifespans in old age.