Decline with old age is a curve. Most people I'd imagine would prefer quality of life to remain high, then drop sharply just before death, to decades of low quality of life. That's what fitness gets you, not longevity.
Also, living a little longer may give you a chance to enjoy what these guys are working on (a cure for aging):
I'll live 50 or 60 or 70 or 80 years and die. You'll live 50 or 60 or 70 or 80 and die. Maybe I'll die a little sooner then you. So what? Maybe you'll get hit by a bus and I'll laugh at all your wasted exercises. I'll have more fun.