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.
