I appreciated the level of thought you put into the argument, even though it does not actually convince me to oppose life extension. Thank you for writing (and prezi-ing) it, I look forward to more.

Basically, the hidden difference if you put me and 9 others behind a veil of ignorance and ask us to decide whether we each get 80 or one gets 800, is that in that case you have the presence of 10 people competing and trying to avoid being "killed" whereas in the choice between creating one 800 year old versus 10 80 year olds is conducted without an actual threat being posed to anyone.

While you can establish that the 10 people would anticipate with fear (and hence generate disutility) the prospect of being destroyed / prevented to live, that's not the same as establishing that 9 completely nonexistent people would generate the same disutility even if they never started to exist.