The question of what people should exist seems complex and subtle, but basic economic theory suggests it may get a lot simpler in the future. Let me explain, via three possible questions.
1. First, consider the binary question, “should creature X exist or not?” Economically, creature X should exist if it wants to exist and it can pay for itself. That is, in a supply and demand world, if our only choice is whether X should exist, then an X that wants to exist should actually exist if its lifespan cost of resources used (including paying for any net externalities) is no more than the value it gives by working for others. To reject the existence of such a creature is to reject an efficiency gain, i.e., a way to potentially make everyone better off. (These costs and benefits are of course marked at market prices. A creature’s value might include donations from other creatures who valued its existence.)
2. Next, consider the question, “Which humans should be created today?” This question is complicated by the fact that each human likely to exist can be created by only one particular set of parents, and then only if the conception lottery goes a certain way. Once a new human exists there remains the question of what resource endowment (positive or negative) the kid should get from its parents. Since this creation scenario is far from competitive, supply and demand doesn’t get us very far in analyzing it.
3. Finally, consider the question, “Which creatures should be created?” in a future where factories can make a wide range of creatures. This situation might arise with whole brain emulation, or advanced genetic engineering. Imagine a supply-and-demand world where many similar competing profit-seeking factories can each make many possible creatures with great precision, endowing them with any preferred debts or rights, but aren’t overly limited by intellectual property rights. When creating creatures is such a competitive industry, supply and demand has strong implications.
All creatures would be created that could clearly pay for themselves (including intellectual property license fees minus existence donations). Since there are vastly more possible creatures than room for actual creatures, costs to exist would be prohibitively high. Most new creatures would have designs near the peak of factory profitability, and own little surplus relative to their cost. Residual control rights (e.g., “are they slaves?”) would rest in the hands of whomever could squeeze the most market value from them. Yes a few would get lucky and become rich enough to have slack for leisure and existence donations; but theirs would be only a small fraction of total wealth. And in a supply and demand world, this distribution of existence, control, and wealth would be Pareto-optimal, economically efficient, and hence good for all the usual reasons.
Another exception to these creature patterns could be due to ancient legacies, of those who held large initial endowments before this competitive regime began. The designs of such ancient creatures, and of new creatures they favored with existence donations, might be unusually far from the peak of factory profitability. Other creatures might question the legitimacy of special creatures who would not exist if not for such legacy assets. They might complain, “Why do such legacies get to be apparent exceptions to the general rule that creatures must pay their way to exist?” Of course if legacy assets were deeply entrenched in social institutions, yet represented only a tiny fraction of wealth, these might remain mere complaints.
Tin-pot dictators and supporting elites often keep their nations poor and inefficient out of (often valid) concerns that efficient economies might no longer tolerate their grabbing such large wealth fractions. Similarly, you might fear you would lose relative power in the above scenario of efficient future creatures. So you might prefer an inefficient legacy control scenario, where your generation coordinates to finely control of all future economies, to be tin-pot dictators of the future. You might try to prevent the creation of these efficient creatures, in favor of creatures you decide should exist, serving you or not as you choose.
If asked what gives you the right to prevent the existence of creatures who could fully pay for themselves, you might respond that you need no right, if you have power and a will to use it. Or perhaps you’ll say ethics assures you it is simply impossible to be unfair to creatures who don’t yet exist. But wearing my efficient economist hat, I cannot support such naked selfish aggression, even if I thought it would work. And knowing how hard is coordination, I have serious doubts re feasibility. If you can identify large negative externalities, I will help you to find ways to price them, to discourage the creation of creatures who cannot fully pay for themselves, and the theft of legacy assets. But if not, I prefer to help creatures who can pay for their existence obtain that exquisite treasure.
Added 1Sep: Let “wants to exist” be “would want to exist if it existed.”