> The children and the elder cannot be sustained by a small number of people of working age

No, the children are fine in this scenario, there are even proportionally fewer than now and so there are any number of available carers.

The elderly are screwed. But, that seems OK?

> If someone things the population on the planet is too big,

This isn't a centrally planned thing, it's just an exaggeration of the observable reality. On the whole humans who could carry a baby to term but understand exactly what's involved are not keen and if they're willing to do it once or twice draw the line there. The assumption that we're just not compensating them financially enough to reproduce more is let's say, not well supported by available evidence.

I think we should choose to be entirely OK with that until there's risk of a real population bottleneck, e.g. 1000x fewer people -- in the expectation that conditions change and it might sort itself out without action.