You said “Ruling out Z first seems more plausible, as Z negatively affects the present people, even quite strongly so compared to A and A+.” The same argument would support 1 over 2.
Granted, but this example presents just a binary choice, with none of the added complexity of choosing between three options, so we can’t infer much from it.
Then you said “Ruling out A+ is only motivated by an arbitrary-seeming decision to compare just A+ and Z first, merely because they have the same population size (...so what?).” Similarly, I could say “Picking 2 is only motivated by an arbitrary decision to compare contingent people, merely because there’s a minimum number of contingent people across outcomes (… so what?)”
Well, there is a necessary number of “contingent people”, which seems similar to having necessary (identical) people. Since in both cases not creating anyone is not an option. Unlike in Huemer’s three choice case where A is an option.
I think ignoring irrelevant alternatives has some independent appeal.
I think there is a quite straightforward argument why IIA is false. The paradox arises because we seem to have a cycle of binary comparisons: A+ is better than A, Z is better than A+, A is better than Z. The issue here seems to be that this assumes we can just break down a three option comparison into three binary comparisons. Which is arguably false, since it can lead to cycles. And when we want to avoid cycles while keeping binary comparisons, we have to assume we do some of the binary choices “first” and thereby rule out one of the remaining ones, removing the cycle. So we need either a principled way of deciding on the “evaluation order” of the binary comparisons, or reject the assumption that “x compared to y” is necessarily the same as “x compared y, given z”. If the latter removes the cycle, that is.
Another case where IIA leads to an absurd result is preference aggregation. Assume three equally sized groups (1, 2, 3) have these individual preferences:
The obvious and obviously only correct aggregation would be , i.e. indifference between the three options. Which is different from what would happen if you’d take out either one of three options and make it a binary choice, since each binary choice has a majority. So the “irrelevant” alternatives are not actually irrelevant, since they can determine a choice relevant global property like a cycle. So IIA is false, since it would lead to a cycle. This seems not unlike the cycle we get in the repugnant conclusion paradox, although there the solution is arguably not that all three options are equally good.
There are some “more objective” facts about axiology or what we should do that don’t depend on who presently, actually or across all outcomes necessarily exists (or even wide versions of this). What we should do is first constrained by these “more objective” facts. Hence something like step 1.
I don’t see why this would be better than doing other comparisons first. As I said, this is the strategy of solving three choices with binary comparisons, but in a particular order, so that we end up with two total comparisons instead of three, since we rule out one option early. The question is why doing this or that binary comparison first, rather than another one, would be better. If we insist on comparing A and Z first, we would obviously rule out Z first, so we end up only comparing A and A+, while the comparison A+ and Z is never made.
The author spends no time discussing the object level, he just points at examples where Scott says things which are outside the Overton window, but he doesn’t give factual counterarguments where what Scott says is supposed to be false.