The main argument is that taking years from potential beings and adding them to existing ones is unjust, hence immoral. Given that, depending on the exact shape of the infinite universes scenario, life extension could be moral, amoral, or immoral. If longer-lived people can reproduce and find new space more quickly than shorter lived people, life extension would be moral. (For example say more experienced people have more motive or ability to create new universes.) However all else being equal (for example, say the limit on reproduction is some unchangeable physical constant that says we cannot make black holes any faster than x, and we have already maxed that out), the fact that shorter lived people are dying and creating spaces for more kids makes that the more moral scenario. While I agree that this is a flaw in the argument (longer lives can possibly result in more new kids born / new spaces opened than shorter ones), I don't think it is my true rejection of the argument overall, because it is not unreasonable to think the new spaces that can be opened is limited and/or cannot be increased by longer lives. I think the real problem is the idea that one can behave unjustly to a person whose existence is only potential, through the act of taking away their existence.

To me, the entire argument sounds like a rationalization for not signing up for cryo. Signed, Someone who has rationalized a reason for not signing up yet for cryo, and suspects that the real reason is laziness.