I don't know if death can really be considered a disease, since it's a fundamental part of evolution. If anything, lack of death could be detrimental to a species.
Natural selection does not imply that the only type of selective pressure is survival. Death is merely one factor that affects reproduction. Further, only premature death matters directly for reproduction. Most people die long after they reproduce (or have had a chance to reproduce).
That's not to say that curing aging won't change the trajectory of the human species' evolution, but to be frank I don't really care about the human species. I care about humans. If someone said: "we should continually kill off the weakest 50% of humans to make the species stronger", I'd say that person is a monster.
Postponing death is a matter of treating diseases and disorders and preventing accidents and violence.
Evolution has non-death mechanisms too; bacteria exchange plasmids. With genetic modifications we're beyond the need for death anyway if we want species-level improvements.
Very importantly: different reproduction rates. This is already more important in human evolution than premature death.