>Centuries ago almost no one lived long enough to die from heart disease, cancer, or dementia. Now those are common, diseases of old age, imposing tremendous costs. I don’t propose going back to 35 year lifespans

This is false. The life expectancy one you reached puberty was very similar to the current one. The 35 years lifespans were due to insane infant mortality. Modern medicine has been able to avoid the death of children but it hasn't being able to extend the duration of your healthy life other than from the statistical point of view.