Starting with a society where people already live to 800 years means starting with a society where those risks have already been mitigated.

Not necessarily at all. Imagine a society that only changed the stuff that requires people dying 1/8 as fast as we did. Imagine they were facing much worse risk of overpopulation, because women could choose to remain fertile for more of their lives. Imagine that some people who wanted to die didn't.

People would STILL refuse to start shooting centenarians. Adult education or drugs that enhance mental flexibility would be better than being shot. Vasectomies would be better than being shot. Allowing voluntary death is better than shooting everyone. Seriously, what kind of person would look at impending overpopulation and go "don't worry about contraceptives - let's just kill 7/8 of the humans on earth."

Heck, we may be facing impending overpopulation right now, depending on what happens with the environment! Should we kill everybody partway through their reproductive years, to avoid it? Of course not! This sort of failure of imagination is a pretty recognizable part of how humans defend privileged hypotheses.