Generally, everyone dies. Traditionally we deal with this by accepting a belief in reincarnation or an afterlife, so it doesn't seem so bad. Or, especially more recently, while you belive death is terminal you learn to accept it as inevitable, and as you get older try to make your peace with it.

Where does this put life extension or cryonics technology? If death may become optional, through technological development, perhaps we should stop accepting it as inevitable? What if we let our dislike of death fuel our researches?

This is the transhumanist position, which Eliezer Yudkowsky espouses in writing after his brother's death:

I wonder at the strength of non-transhumanist atheists, to accept so terrible a darkness without any hope of changing it. But then most atheists also succumb to comforting lies, and make excuses for death even less defensible than the outright lies of religion. They flinch away, refuse to confront the horror of a hundred and fifty thousand sentient beings annihilated every day. One point eight lives per second, fifty-five million lives per year. Convert the units, time to life, life to time. The World Trade Center killed half an hour. As of today, all cryonics organizations together have suspended one minute. This essay took twenty thousand lives to write. I wonder if there was ever an atheist who accepted the full horror, making no excuses, offering no consolations, who did not also hope for some future dawn. What must it be like to live in this world, seeing it just the way it is, and think that it will never change, never get any better?

You can see this also in his Harry Potter and the Methods of Rationality, when Harry qrfgeblf n qrzragbe. [1]

Another person taking a similar view is Aubrey de Grey, an anti-aging researcher. When asked about existential risk, the possibility of something that might end humanity, he replied

I've never been convinced that the elimination of humans all in one go is all that much worse than the elimination of humans on a steady schedule of 150,000 per day. The only difference seems to be that in the latter case there get to be more people in the long run [2] - but so what?

While I'm not pro-death, I now see I'm not all that anti-death either. A death is bad because of the effect it has on those that remain and because it removes the possibilty for future joy on the part of the deceased. A world in which we have ended death but are still limited by the carrying capacity of Earth means the same number of people living at once as one in which people still die and are born.

This may be better than the world now, but I could also see it being worse. On one hand, not having to see your friends and family die, increased institutional memory, more time to get deeply into subjects and acheive mastery, and time to really build up old strong friendships sound good. But dramatically fewer children? Much less of the total human experience spent in early learning stages? Would we become less able to make progress in the world because people have trouble moving on from what they first learned?

So I'm not sure whether extending the human lifespan is valuable, but I think "defeating death" is only as important of a goal as the amount a longer life is better, per year, than a shorter one.

(Julia Galef looks at this on lesswrong and her blog, but more from the perspective of defending and explaining the transhumanist perspective on death.)

[1] In Chapter 45, especially. But if you've not read it before, much better to start at the beginning than read that chapter in isolation.

[2] This is where I have very different values: "more people in the long run," in the sense of more person-years, is quite a good thing, not at all a "so what?". Cutting off all the potential happiness of future people is incredibly bad, which is what makes existential risk such a problem.