This is true, but it's also possible that the very treatment that supposedly decreases cancer deaths also decreases the "other illnesses" such that the distance

This is true, but it's also possible that the very treatment that supposedly decreases cancer deaths also decreases the "other illnesses" such that the distance between the cancer death and other illness death is constant. In this case, lifespans could increase and still result in other illnesses killing more than cancer in spite of increased life expectancies. Either way, we are living longer though.