Your claim doesn't argue against the issue. Even if we accept that you're correct there, you're again speaking of more people getting to their 'expiration date' rather than expanding that date itself. If you cure cancer, heart disease, and everything else - we're still not going to be living to a 100, or even near it, on average.

The reason humans die of 'old age' is not because of any specific disease but because of advanced senescence. Your entire body just starts to fail. At that point basically anything can kill you. And sometimes there won't even be any particular cause, but instead your heart will simply stop beating one night while you sleep. This is how you can see people who look like they're in great shape for their age, yet the next month they're dead.