Doesn't that argument prove too much? In the past, people's lives were shorter and more full of misery. Every improvement we've gotten has been due to people rejecting your philosophy. Assuming one can live longer (without being frail and demented) why not?

It seems like if most people could choose, they'd prefer to have the mental and physical abilities of a 20-something while living as long as they wanted. Eventually, medical technology will get to the point where that is possible. Why not hasten such a day rather than let more people get old and frail? Yes, people will still eventually die, but living for 300 years in peak condition seems better than the status quo.