Nothing says that we are unable to do these projects at the same time. The people who could work on anti-aging medicine aren't interchangable with the people who are working on various aspect of moving society toward an environmentally sustainable society. Otherwise this is the same argument being made against NASA and research into rocketry and spaceflight.

Not dying due to cancer at 20 and living to be 400 years old isn't quite what i would consider in the same realm of justification, but your opinion might differs.

Really? Someone's going to cry when their loved one die. The older an individual is, the greater their network of connections, knowledge, skills, and lived experience. All of which are valuable to societies.