This fails to take into account the difference in quality of life between a brain devoting entire stars or galaxies to fun and a human living in a poor region of Africa (as you acknowledge when you say people don't base their decisions solely on "years lived"). On the other hand, there is also the point DanielLC brought up.

I don't think the rough equality of the number of expected life-years saved reflects anything interesting. Eliezer's estimate could vary by many orders of magnitude even if its basic assumptions were true. The probability estimate could also vary by orders of magnitude.