I studied philosophy—but I don’t get the argument. Furthermore, I don’t think there’s any such X such that X resolves population ethics.
(The following is aimed mostly at people who don’t see a problem with the OP).
The problem in population ethics are usually phrased as impossibility theorems: Desirable and intuitive properties for which we know we can’t have all of them (here the statement of an older one, see also Arrhenius 2000 for a longer exposition).
Thank you both for your comments. Let’s consider an example.
Suppose you are tasked with ethical decision-making and part of a moderately sized population on a moderately sized island. Moreover the current population size is exactly at the minimally viable population size to maintain sufficient genetic diversity to sustain itself into the future. It is also exactly at the carrying capacity, such that growth in population will be followed by scarcity and population decline, after which the population will be below the minimally viable population size. There is no expectation of additional lands or technologies (to effectively increase the carrying capacity) in the foreseeable future.
Would it in this case be ethical to advocate for a change in the population size? And if the question of population ethics can be answered in this particular case, the statement that the question of population ethics cannot be answered in general must I think be false(?).
Moreover, my argument has an if-then structure and conditions on a belief in (strong) longtermism. If you believe in (strong) longtermism, you care about the future, so it seems to me the answer to my above question should then be no; in order to have a sustainable population size, and safeguard the potential in the future.
For example in this way, I state that (strong) longtermism restricts the space of permissible positions in population ethics, with those positions remaining that minimize the risk to the potential in the future. This could still be multiple or even many permissible positions, such that the issue is not quite resolved yet. But to a (strong) longtermist it would be resolved in the sense that the different options would not be relevant to decision-making, as they would care only for and prioritize the potential in the future (such that they are indifferent between these multiple permissible positions).
To think about possible worlds, as philosophers do, is then valuable in a developed state of many options, as it helps to triage among them. In my example here—a state and actual world (within a thought experiment, ironically) of few (or even not multiple ethical) options—this is however not the case.