The longer a plan takes, the more things will change before it comes to fruition. The world will be different tomorrow, and different again each day thereafter. Therefore, it is perfectly rational to make plans for the near future, but to wait for more information before making plans for the distant future. We can argue about the precise degree to which we should favor the short term vs. the long term in our plans, and there are some reasons to think that people have it mis-weighted, but simply telling people to "favor the long term more" won't do because it's possible to overshoot and neglect the present.
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.