Overpopulation is one of the great apocalyptic fears of our era, and like many of the rest of those fears, it is unfounded. If anything, the trajectory is for increased wealth to reduce population as children shift from being a necessary benefit to what is effectively a form of luxury good. Further, the effects of great longevity on population size are generally much smaller than people imagine, as rigorous modeling shows. Lastly, this planet could support more than ten times the current population using present day technologies, were more of the land used efficiently. Yet still, people look at the results of war and kleptocracy and cry overpopulation rather than recognizing the true causes of suffering.
In this article, I'll try to show that the overpopulation objection against rejuvenation is morally deplorable, that not developing rejuvenation for the sake of avoiding overpopulation is morally unacceptable, and thus overpopulation doesn't constitute a valid objection to rejuvenation. I'll start with an example. Imagine there's a family of two parents and three children. They're not doing too well financially, and they live packed in a tiny apartment with no chances of moving somewhere larger. Clearly they cannot afford having more children, but they would really like having more anyway. What should they do? The only reasonable answer is that they should not have any more children until they can afford having them. Throwing away the old ones for the sake of some other child to be even conceived yet would be nothing short of sheer madness.
That being said, let's have a look at the overpopulation objection. It can be summarised as follows: If we cured ageing, we would end up having more people than our planet can sustain. Therefore we should not cure ageing. Translation: Curing ageing means eliminating age-related diseases as a cause of death, i.e. eliminating a very effective way to get rid of older people. If we don't get rid of older people, we won't have room for new ones, so we shouldn't cure ageing.
If we were to apply this logic to the small-scale example of the family, we should get rid of the older kids to make room for the new ones, and I'm not talking about kicking them out of the house when they're 18; new people are born all the time in the world, which in our small-scale example translates to the family wanting more kids here and now. Not curing ageing means letting people become sick with horrible age-related diseases and die of them; in the small-scale example, this could be compared to not vaccinating the kids. It goes without saying that, from a merely moral standpoint, if we're afraid that we might end up having more people than we can afford having, the appropriate answer to this problem is 'let's not make more people than we can afford having'.
Still, the idea of present-day people dying for the sake of potential future children who aren't even in their mothers' wombs yet somehow seems perfectly acceptable; however, when applied the example of the family, the very same idea appears to be a clear case of being several sandwiches short of a picnic. Why this double standard? I can think of three reasons. The first, obvious reason is that death by ageing happens 'naturally' and up until now has been inevitable, so by the false equation 'normal'='right', people conclude this is how things should be. The second reason might be that we don't value elderly lives as much as children's. This may be understandable from the cynical survival-of-the-species point of view, but is absolutely undefendable from any humane point of view. The third reason is that we don't really think of humanity as some sort of big family. The children of the family example are much more 'concrete' than the elderly people of the large-scale example. When you think of the former, you identify with one of the parents and are horrified at the thought of throwing away your own children; when you think of the latter, for the most part they're just random elderly people whom you don't know and have no emotional attachment to.
Long story short: You can't use overpopulation as a reason to object to rejuvenation biotechnologies, because you can't ask people to give up on good health and potentially indefinite lifespans for the sake of people who aren't even in the making yet. The only reasonable alternative is that we don't make more people than we can afford having.