Imagine if we, somehow, had the technology to make humans live for 5000 years (bristlecone pine trees somehow manage it). Would you then say that we should still commit suicide at 90, in order to make room for the young?

My dad is in his 70's, and I consider his mortality to be a preventable tragedy, like a death from smallpox before the invention of the smallpox vaccine.

There's already a lot "unnatural" about humanity (unnatural only if you don't think of humanity as being part of nature). It used to be the case that about half of your siblings would die before reaching adulthood. Perhaps at the time, people felt that this gave their own life more meaning, and perhaps in some sense, it did. I'm still glad we don't have to go through that anymore, and grateful to generations past for spending the resources to make that so.

Children would have to be much rarer if we solve longevity, but our engineers and scientists would also be far more capable than they are right now, as they'll be able to accumulate far more knowledge and experience. Perhaps, once we solve longevity, it will be time to start the long term project of colonizing the solar system.