Modern rich humans live 80 or so years, but most of us have great difficulty planning (or following plans made for) more than a few years or maybe a decade out.
I don't think failure to think long-enough term is caused by our limited lifespans, so increased lifespan is probably not sufficient to improve it. In fact, the opposite is likely true: having longer views and considering multi-decade effects of today's action is likely to RESULT in longer lifespans.
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.