One ethics-based argument against radical life extension I've encountered is that the notion of vastly extending one's lifespan is somehow "selfish". The way I see it, wanting to keep living when you're 120 is no more selfish than wanting to continue living when you're 12. I think it's a very, very dangerous line of reasoning that could follow from making the "it's selfish" argument -- basically, this argument assumes that one person is justified in saying when another person is obligated to stop existing.
To illustrate this point, If Bob wants to live forever and will do what he can to achieve it (so long as he remains informed the entire time and does not knowingly or deliberately influence the world in a negative manner), why should Joe be able to say, "Bob, I'm sorry, but there is going to come a point where the best thing you can do for the world is die". I don't think that Joe has any real ability to predict the ramifications of Bob's continued presence. What if Bob is a tireless altruist, working to improve conditions in developing nations? What if Bob is an inspiring poet whose works move people to new heights of understanding of their own condition? How can Joe possibly insist that no matter what Bob is doing, his mere presence beyond a certain age is nothing but a detriment?
I would argue that even if Bob is just an "ordinary" person, living a quiet life in the country, planting his vegetable garden and reading the paper and spending time with his cats, he still has the right to maintain this existence for as long as he would like to. It's not up to Joe to make the decision or judgement as to whether Bob should get to continue living. It's actually more selfish (or at least self-righteous) for Joe to insist that Bob's lifespan should be limited by Joe's philosophy. If Joe wants to die, and feels it's somehow his duty to die, then that is his choice, but it doesn't have to be Bob's choice.
Joe will argue that Bob's continued presence represents an unfair use of resources, that Bob's long life somehow interferes with the future by denying someone else -- who hasn't been born yet -- the chance to exist. The thing is, there is no proof that Bob's life in any way prevents anyone in the future from existing. For all we know, Bob's presence could be granting others MORE opportunities. If Bob (at a robust 200 years of age, perhaps?) tutors his niece in some aspect of art or science and this niece goes on to enrich the lives of others, then couldn't it be argued that Bob's presence -- at age 200 -- actually improved the world, rather than made it worse?
Wanting to live a very long time may be "selfish" in the sense that most human endeavors are selfish in some way -- they allow continued surivival, and opportunities, that may or may not impact the survival and opportunities of others. Driving a car (rather than taking public transit or walking or biking) could be said to be "selfish" because it only behooves the driver's convenience, and inarguably has some sort of environmental impact. Grocery shopping for yourself and not buying anything to donate to a homeless shelter could be considered selfish. Having one's own children rather than adopting (especially when you're concerned about overpopulation) is almost inarguably a selfish act. None of these things are immoral in my estimation, because like it or not, what we call "selfishness" is inseparable from living. So perhaps life extension is selfish, but it's no *more* selfish or likely to result in the breakdown of society or the environment than anything else many people do, (and are not bothered about to any real extent).
Another reason I take issue with the "selfishness" argument as applied to life extension is that it has implications for such things as disability rights. To what extent does a person have to justify their own existence, present or continued? If one claims that people beyond a certain age are obligated to die (because they've somehow used up "their share" of certain resources), then it seems that this involves making a judgement as to what constitutes a contribution to society. What about a disabled person -- someone who is perhaps unable to walk, or cook without assistance, or even move? Is Stephen Hawking using up "too many resources" because he requires a wheelchair, other assistive technology, and staff persons to help him survive? Think of all the people that could be fed, clothed, etc., with the money and time resources it takes to keep this one man alive...does that mean that Dr. Hawking should feel guilty for merely existing, and that the world would be better off without him?
Most people reading this would probably say no, that Dr. Hawking should not feel guilty about his needs because he is making a tremendous contribution to society. But what if he wasn't? What if he was just a disabled man, with no PhD, who needed the same supports to survive but was not a physicist.
Would he then be obligated to die?
Again, I say absolutely not. He would still be a person, and it is impossible (not to mention immoral) for an outside agent to even attempt to judge whether he has a right to continued existence. It sets a very dangerous precedent when one starts using the resource / social contribution model as a rationale for encouraging or disagreeing with the maintenance of people's lives. Lots of people who are outside the visible economy make incredible contributions to human existence, yet are not necessarily recognized or acknowledged for doing so.
There are retired senior citizens who perform volunteer work, which represents a tremendous amount of "free labor", which most certainly benefits the world. There are disabled individuals who cannot hold paying jobs, but who write powerful and passionate essays on human rights and share them with the world through the computer and other means of publication. The point I'm trying to make is that resource use can be a good investment in many cases, and that one needs to be on the lookout for "contributions to society" where one might not expect them, because when contributions are atypical or not subject to direct economic measurement, they can simply "pass under the radar" unless one is looking for them.
I agree that society does benefit from a certain amount of turnover and refreshment of perspectives, and that if everyone alive today magically just Didn't Die (yet we kept reproducing at the same ridiculous rate) there would be serious problems...but still, this doesn't convince me that it's unduly selfish to live or keep living. People are often called "selfish" if they commit suicide, and yet people are also called "selfish" when they endeavor not to die at all. One's presence, or absence, can affect the world (and the Universe) in completely unpredictable ways. Rather than assigning the "selfish" moniker to life-extensionists, perhaps it would be more rational to just keep applying it to suicides, since the argument used to justify the labeling here is that those who off themselves are denying the world of their contributions.
It's evident that this reasoning could just go round and round and round.
Therefore, it seems that the MOST pertinent thing to do is not worry so much about whether other people are doing things, or holding philosophies, that you consider "selfish". Rather, each person should be free to pursue a life that they are suited for, and work toward goals that are meaningful for them. Whether they look impressive to anyone else or not.
It's all well and good to talk about the "greater social justice", but if your favorite relative was 95 years old, in some sort of terrible pain, and DID NOT want to die, I can't imagine anyone here would sit there and say, "Well, Grandma, too bad...you've had your chance to live, and now you need to get out of the way so your presence doesn't somehow deny arbitrary future people the right to exist." I think it would be sick and reprehensible to sit there trying to convince such a person that they shouldn't desire to live any longer, or withhold medical treatment from them on the basis that they were "using up resources".
After all, it is one thing to say on the statistical, impersonal level, "Well, a certain number of people have to die every year in order to maintain the social order and natural equilibrium". It's quite another to say that someone's relative, or YOUR relative, "deserves" to die. Even if they'd rather not. Even if they haven't finished experiencing things and forming connections between thoughts, etc. Maybe a lot of people just accept death, but those that do not should NOT be obligated to experience decline and suffering. And they shouldn't be made to feel guilty for existing, regardless of how old they are.