1. One could use the classic utilitarian argument in favor of equality: diminishing marginal utility. However, I don't think this works. Humans don't seem to experience diminishing returns from lifespan in the same way they do from wealth. It's absurd to argue that a person who lives to the ripe old age of 60 generates less utility than two people who die at age 30 (all other things being equal).

I'd say that giving all the doses to one person results in less marginal utility per year than dividing the doses among the population, because when one person receives all the doses, they take a hit to the average utility of each of their years by outliving everybody they've ever known and loved.

I would certainly rather live 999 years along with the people I care about than 1000 years, during which I have no peers or loved ones who share comparable lifespans.