> that issue has been solved

It depends on what you consider the issue to be. If anyone is both an environmentalist/conservationist and honest about it, a reduction in population would be a good outcome for biodiversity and would permit rewilding land currently used for agriculture, housing, commerce, etc. In the context of our current economic systems, a declining population is a disaster waiting to happen.

Some people may have considered limiting children to one per family to be "beyond government control", so there's probably some good sci-fi to be written/lived through on the topic of what a totalitarian government will get up to when it's realised that more children are "needed", but that most parents won't volunteer to provide this service.