People still live only around 100 years or less. Saving the planet doesn't mean anything to the dead. Besides, if you can't save the planet (which you can't), why waste time on it? First learn how to live potentially for 1,000 years. Then, after 200 years, you might gain enough wisdom and knowledge to do a little to save the planet. 
According to the scientists, the theoretical maximum life expectancy is around 125 years. This would mean that irrespective of the advance in medical science, the human lifespan is unlikely to increase much further.
Over a hundred years ago scientists said they knew everything that there was to know. That was before rockets went into outer space. That was before we had atomic power and cars all over the place. No heart transplants back then.
Why would they think that they have figured it all out now?