As I understand, the question was about life extension, which does not equal immortality. I have heard a few plausible arguments against immortality, but here it seems that those people are arguing that if we had to choose between living for 80 years and 150 years, we should take 80, for sophisticated reasons. An that is stupidity.
And I don't think that the general audience is firmly entrenched in sophisticated positions. Most people would gladly accept that the moral question about death is simple. We see few people arguing that cancer is good, even if many those sophisticated arguments for death could be used to support cancer as well. I haven't also ever seen somebody to argue, on whatever level of sophistication, that e.g. Japan is inferior to Zimbabwe because people in Japan live longer. Once a longer life becomes a real option, everybody will choose it if he can. When people rationalise (early) death as good, it is only because they see the alternatives as only hypothetical.
Once a longer life becomes a real option, everybody will choose it if he can.
That is not what currently happens. There are big differences in the way people age, and when they die. Many of them can be linked to lifestyle decisions, some are well researched, and a few even widely known due to public propaganda. And still people smoke. If a quick/fix, a pill, o something that looks like treatment becomes available that might change. But at the moment much of what actually can be done to improve life quality, length, or probability of surviving is just not done.
Last Wednesday (2010 Dec 01), BBC Radio 4 broadcast a studio discussion on the question: "should we actively try to extend life itself?" The programme can be listened to from the BBC here for one week from broadcast, and is also being repeated tomorrow (Saturday Dec 04) at 22:15 BST. (ETA: not BST, GMT.)
All of the dreadful arguments for why death is good came out. For uninteresting reasons I missed a few minutes here and there, but in what I heard, not one of the speakers on any side of the question said anything like, "This is a no-brainer! Death is evil. Disease is evil. The less of both we have, the better. There is nothing good about death, at all, and all the arguments to the contrary are moral imbecility."
Instead, I heard people saying that work on life extension is disrespectful to the old, that to prolong life would be like prolonging an opera, which has a certain natural size and shape, that the old are wise, so if we make them physically young then old people won't be old, so they won't be wise. Whatever cockeyed argument you can construct by scattering into a Deeply Wise template the words "old", "young", "wise", "decrepit", "healthy", "natural", "unnatural", "boredom", "inevitable", "denial", I heard worse.
If I can bear to listen again to the whole thing just to check I didn't miss anything important, I may write something on their discussion board.