Most people in their 80s and 90s have cancer, but thanks to ageing the cancer is very slow growing so they die of something else. For example, in men the percentage probability of having prostate cancer is equal to your age yet few men die of prostate cancer.

There are some mutations in people that slow ageing at the expense of increasing the cancer rate. There is a very interesting one from Brazil where a mutation in the p53 gene has this exact mechanism [0].

0. https://youtu.be/URKJ7LLXc3E (you can skip the first 5 minutes - the relevant part is around minute 15).

I think you mean, most people in their 80s and 90s have benign tumors, not cancer.

Heart disease research investments would likely have equal or superior payoffs to cancer research investments.

No they have full non-benign cancers, they are just so slow growing that the person with them normally dies of something else first although plenty of people in their 80s and 90s die of cancer.

Yes the life expectancy increase from curing cancer is only around 2 years, but it is the essential first step to doing something major about ageing.

non-benign cancer = invasive tumor. people with invasive tumors rarely die of other causes which are not correlated.

anyway, I don't think anybody in the serious scientific community believes that a cancer-only research program would have a huge impact on longevity and instead, most people advocate for a portfolio with roughly 70% spent across cardiovascular and cancer, and the rest on other causes.

Heart disease and cancer are symptoms of aging, not causes. Why would we treat the (very many) symptoms individually if we can treat the cause?

do you know the cause of aging? Nobody does. Instead, what we do is address the symptoms, because that generally leads to longevity and higher quality of life in older years. That's what the medical system is dedicated to doing. I say this with full knowledge that my friends who work at Buck Institute of Aging are working on the underlying causes of aging and they've all said it's better to focus on treating those symptoms.

BTW what you've said is also fairly philosophical. It would be completely correct to say that heart disease is a cause of aging, under a reasonable definition of aging.