But in my thought experiment, average utility remains unchanged.
The average utility, counting only those two people, is unchanged (as long as we assume that life from 0-100 is as pleasurable as life from 100-200). But firstly the utility of other humans should be taken into account; the loved ones of the person already living, the likely pleasure given to others by younger people in comparison to older people, the expected resources consumed etc.
But perhaps your thought experiment supposes that these expected utility calculations all happen to be equal in either case, too. In that case, surely there is still your own utility to be taken into consideration!
I don’t know much about utilitarianism (generally I regard all attempts at inventing moral formulae to describe human values to be hopeless, because we aren’t as a matter of fact reflectively consistent), but presumably in utilitarianism you are allowed to have aesthetic values apart from the value attached to pleasure and suffering in others (otherwise I conclude that utilitarianism is one of the less sensible such formulae). Therefore if the utility of others is the same either way, your personal non-altruistic aesthetic values are the deciding factor (and the question of how these are to be balanced with the value attached to the utility of other people is irrelevant in this case). Clearly your non-altruistic values do prefer the idea of life extension, so I don’t see any problem here.
The average utility, counting only those two people, is unchanged (as long as we assume that life from 0-100 is as pleasurable as life from 100-200). But firstly the utility of other humans should be taken into account; the loved ones of the person already living, the likely pleasure given to others by younger people in comparison to older people, the expected resources consumed etc.
But perhaps your thought experiment supposes that these expected utility calculations all happen to be equal in either case, too. In that case, surely there is still your own utility to be taken into consideration!
I don’t know much about utilitarianism (generally I regard all attempts at inventing moral formulae to describe human values to be hopeless, because we aren’t as a matter of fact reflectively consistent), but presumably in utilitarianism you are allowed to have aesthetic values apart from the value attached to pleasure and suffering in others (otherwise I conclude that utilitarianism is one of the less sensible such formulae). Therefore if the utility of others is the same either way, your personal non-altruistic aesthetic values are the deciding factor (and the question of how these are to be balanced with the value attached to the utility of other people is irrelevant in this case). Clearly your non-altruistic values do prefer the idea of life extension, so I don’t see any problem here.