I think the fundamental misunderstanding is that most people think that people die from diseases, but in reality it's much closer to the other way around; people get diseases when they are about to die. Even if we had the cure to every single cancer the average life expectancy would only go up by about 3 years, because most cancer patients would just die from something else anyway.
True but overly defeatist. Fixing one problem would not solve things. Fixing several problems (cancer, intra-cellular junk, extra-cellular junk, etc etc, Google "SENS" for a suggested list) would actually cure death from ageing - or at minimum, would reveal what other problems we would also need to solve. People get sick because they are damaged, and they are damaged because they are old, but the damage is in principle something that can be repaired.
You make an important point: treating ageing itself is superior to tackling diseases individually. Ageing is a risk factor for practically every major disease. Therefore reversing, halting, or even just slowing the process would reduce the incidence and burden of every major disease.
