For those who believe in God, why do you think <he> gave humankind a brain? Assuming we have freewill, with an omniscient supreme being, don't you think this was anticipated? If the capability exists within the human physiology to support extension of a lifespan, I find it trite to argue against it because of finances (i.e. Social Security), the job market, or overpopulation. It is akin to saying why bring a baby into a world where you know he/she will experience hunger, pain, sadness, disappointment, violence, etc.
The Earth's ecosystem will adapt over a period of time to whatever we do, and so must we or become extinct.
I don't see the cause for fear here.