We don't know how to reliably plan for long term. We didn't have enough iteration of "long term" in any environment similar to our current environment to have any hope for long term planning to work. Try guestimating 2009 based on data from 1909 alone and tell me how it went. Or 2009 based on 1959.
Even the most basic things like human population, and GDP/capita cannot be estimated with anything remotely resembling reliability past a couple of decades.
One of the proposed benefits of life extension is that it will help us long term plan as we will be around in the future, so we will be more likely to care about the long term future of the world if we live longer.
So is this true? Are we rational in this respect or will the mind recoil from thinking in time scales longer than 40-60 years even when we are living hundreds of years, due to biases intrinsic to the mammalian brain.
I don't have time to research this question right now, so I thought I would experiment by throwing out this question to lesswrong and see how people treat it.