If I had to choose a society with all young people, all old people, or a healthy mix of each... i'd choose the later. Young people provide new thinking, they're essential to society. Old people provide foundation, and wisdom, they're essential too. We live on a planet with finite resources, to support the right mix, we should accept that its just not RIGHT to live forever, even if it were possible. I'd like to live until i'm 100, and I hope science makes it so i'm just as productive at 100, as I am today.
If society included lots of immortals, they'd be pretty driven to make sure that we're not stuck living on a planet with finite resources. There are plenty of untapped resources laying on the ocean floors and deeper into the Earth's crust than we typically mine today, and lots more off-world. Those are all technologically within our grasp today, they're just not cost-effective. But if we had to support an immortal society, the ROI terms change: it becomes reasonable to invest in an infrastructure that will take a few centuries to pan out, because people will live to see it completed and get the benefits from it.
Re: finite resources - Constraining the human race to a single planet puts a pretty small upper bound on our species' potential.
Humans evolved a desire to explore and adapt to new environments as an evolutionary advantage, this advantage doesn't need to be bound to one tiny little ball floating in space.
I assume there are only 3 sustainable paths for the human race. The first, is the norm. We continue as we are, we don't adjust to climate change etc in time. Society eventually collapses under the magnitude that it has swelled up to. We enter a second "dark age". The second, is we find a way to to sustain with the resources we have on this planet alone. Population finds a balance, and we go on living another 5000 years, just like we did for the first... except now technology plays a bigger role. The third, is we expand to space and limits become boundless.
The only scenario where we can think about super long life spans is the later. My point is, this is like deciding how to spend your millions of dollars before you've made them. My personal opinion is that the singularity is as close to inevitable for as long as we as a society choose to pursue technology. To me the likely scenarios are either A or C, and it really depends on which happens first... an environmental collapse or the singularity.
