Yes, I took it that you were agreeing with
@slant. I was playing around with options really rather than making hard suggestions, and printing money would not work. What I'm driving at is that in developed countries we have made it unattractive for youngsters to have children.
slant was suggesting that banning abortion would help increase the number of babies born, but it comes with baggage of the sort I mentioned. If governments intervene at all, it almost certainly would have to be like this sort of thing, or they would have to make having more children more attractive to young folks and not having children less attractive. For example if you made it financially more attractive to have more kids, the money would have to come from significantly higher taxation on those who had none or just one. There are other things you could do too, such as restricting residence in desirable properties to families with more kids. That would have to persist until the current culture all over the world that's leading to this problem is changed - it can be done because the video slant posted shows how effectively China got people to embrace one child families using these sort of techniques and now can't reverse it. It'd be wildly unpopular though and political dynamite.
But I wonder if we should really worry too much about this. You mentioned the fear of environmental crisis as putting people off having a family - but it seems very likely that if the world ended up in a couple of generations with a lot fewer people, then combined with new technology this could put a lot less stress on the environment and could actually be of very great long term benefit. OK there is a transition when there are a lot more older folk than young ones proportionately, but there is a clear solution to this - time. We might go through some tough decades, but in the end things will rebalance as the excess of older folks die off.
Another thing that strikes me is that ordinary evolutionary pressure might take over in a couple of generations. It will be those people who still want a lot of children now despite the prevailing culture who will be most successful at passing down their genes, and that may well bring about new generations of folks who want more kids than at present.