You're right homewithaview, re ageing populations. But, since we, in the UK, where the birth rate really isn't high, are still nowhere near one child per couple, (we are higher; at 1.8, I believe?), I don't think there is much danger of us becoming like China. Although some people are concerned about the ageing population here already.
However, as people always say on these sorts of threads, having more babies to care for the elderly becomes a viscous circle as those babies grow up, become elderly, need to be cared for, so we need more babies to care for them and so on and so on. So, ideally, we would find a way to do with gradually fewer young people caring for older generations, so when they grow up and become elderly, we can do with fewer young people and so on. But that would mean accepting a stagnant or even shrinking economy, which is a big issue. Because, as you rightly say, it isn't just being cared for physically, in old age, but that we rely on younger people to run public services when the elderly population no longer can. So, things like hospitals, emergency services, agriculture etc, all need young(er) peopleto run them.
I personally feel that as CC worsens, environmental politics will become more and more mainstream (as it already is). When it takes centre stage, growing economy / GDP will no longer be the way countries tend to measure their success, so smaller populations will be more acceptable.
Also, they didnt "try this" in China. I am talking about couples who care a lot about the environment, but who still want children, choosing not to have more than one or two. In China it was obviously ghastly, with forced abortions, baby girls being abandoned or aborted because of the preference for boys etc. I don't agree with forcing couples to limit the number of children they have for the sake of the planet, but I do think many couples choose to do it of their own accord.
But you're absolutely right to say that it is more complicated than having babies is 'bad' and the fewer the babies born the better.
Sorry if this is a ramble; this is one of my 'pet subjects' 
. I am a horrible bore...
.