I think the examples you use to motivate the intuitions are doing a poor job, however, because they involve too many other complicating factors.
That's deliberate, for two reasons. The first is that I think that some non-individual preferences are relevant to population ethics. I don't see them playing a strong role, but I do see them existing - that, say, a fully equal population is at least somewhat better than one with same total happiness and great inequality. Given that those are my preferences, I like to include elements which I feel may have such impacts in my descriptions. People who see only personal preferences should not be affected by the way I phrase the descriptions (just the ultimate happiness/utility/preference satisfaction). And it would be useful for others to see a more complicated and evocative image of how things go, which may trigger new intuitions.
But the main reason I try and do it is because I feel that the debate has gotten too clinical - that we throw around terms like "barely worth living" without really connecting to what they mean. "Barely worth living" is, by modern western standards, really awful - a young girl born into a rural village, fell in love with X at 14 but was raped by Y that year and made to marry him. Working on the land every day with her fading strength eyesight, she found some pleasure in her (very few) surviving children, but they all ultimately pre-deceased her, her favourite son being gutted before her by invaders, who then raped her again and broke her legs. Unable to work much after than, she took to begging in a new village, and found some solace in religious belief and in playing the role of the madwoman, but ended up dying terrified, alone and in great pain.
Or something along those lines, depending on where exactly you put the zero. I think that people who are in favour of the repugnant conclusion should be willing to face exactly what it means to have a life just above zero - if you inflicted that fate on most people today, you'd get deservedly reviled and imprisoned. I can see the worth of being dispassionate in making decisions, but not in choosing your values.
Secondly, introducing such a population very likely has negative externalities, making the lives of those already living in the society worse
A very valid point, though my gut feeling was that it would have positive externalities as well (people like to feel superior). You can reduce the negative externalities by making the new population docile and unwilling to revolt, and reducing the cross-population empathy. Problem solved! (contrast this with my first point above).
if you inflicted that fate on most people today, you'd get deservedly reviled and imprisoned.
I find that a rather odd statement. Isn't "zero welfare", by definition, the amount of welfare such that any life with greater welfare is a life which it is good to create? It seems like there's a paradox in the conception of "zero welfare". We wish to define such absolute minimum amount of acceptable welfare, but the idea of subjecting someone to the bare minimum of welfare is repugnant.
It's like those "structuring" laws, where it...
This is not the post I was planning to write. Originally, it was going to be a heroic post where I showed my devotion to philosophical principles by reluctantly but fearlessly biting the bullet on the sadistic conclusion. Except... it turns out to be nothing like that, because the sadistic conclusion is practically void of content and embracing it is trivial.
Sadism versus repugnance
The sadistic conclusion can be found in Gustaf Arrhenius's papers such as "An Impossibility Theorem for Welfarist Axiologies." In it he demonstrated that - modulo a few technical assumptions - any system of population ethics has to embrace either the Repugnant Conclusion, the Anti-Egalitarian Conclusion or the Sadistic conclusion. Astute readers of my blog posts may have noticed I'm not the repugnant conclusion's greatest fan, evah! The anti-egalitarian conclusion claims that you can make things better by keeping total happiness/welfare/preference satisfaction constant but redistributing it in a more unequal way. Few systems of ethics embrace this in theory (though many social systems seem to embrace it in practice).
Remains the sadistic conclusion. A population ethics that accepts this is one where it is sometimes better to create someone whose life is not worth living (call them a "victim"), rather a group of people whose lives are worth living. It seems well named - can you not feel the top hatted villain twirl his moustache as he gleefully creates lives condemned to pain and misery, laughing manically as he prevents the intrepid heroes from changing the settings on his incubator machine to "worth living"? How could that sadist be in the right, according to any decent system of ethics?
Remove the connotations, then the argument
But the argument is flawed, for two main reasons: one that strikes at the connotations of "sadistic", the other at the heart of the comparison itself.
The reason the sadistic aspect is a misnomer is that creating a victim is not actually a positive development. Almost all ethical systems would advocate improving the victim's life, if at all possible (or ending it, if appropriate). Indeed some ethical systems which have the "sadistic conclusion" (such as prioritarianism or egalitarianism) would think it more important to improve the victim's life that some ethical systems that don't have the conclusion (such as total utilitarianism). Only if such help is somehow impossible do you get the conclusion. So it's not a gleeful sadist inflicting pain, but a reluctant acceptance that "if universe conspires to prevent us from helping this victim, then it still may be worth creating them as the least bad option" (see for instance this comment).
"The least bad option." For the sadistic conclusion is based on a trick, contrasting two bad options and making them seem related (see this comment). Consider for example whether it is good to create a large permanent underclass of people with much more limited and miserable lives than all others - but whose lives are nevertheless just above some complicated line of "worth living". You may or may not agree that this is bad, but many people and many systems of population ethics do feel it's a negative outcome.
Then, given that this underclass is a bad outcome (and given a few assumptions as to how outcomes are ranked) then we can find other bad outcomes that are not quite as bad as this one. Such as... a single victim, a tiny bit below the line of "worth living". So the sadistic conclusion is not saying anything about the happiness level of a single created population. It's simply saying that sometime (A) creating underclasses with slightly worthwhile lives can sometimes be bad, while (B) creating a victim can sometimes be less bad. But the victim isn't playing a useful role here: they're just an example of a bad outcome better than (A), only linked to (A) through superficial similarity and rhetoric.
For most systems of population ethics the sadistic conclusion can thus be reduced to "creating underclasses with slightly worthwhile lives can sometimes be bad." But this is the very point that population ethicists are disputing each other about! Wrapping that central point into a misleading "sadistic conclusion" is... well, the term "misleading" gave it away.