I don't think we can do much and I am not sure we
should do much either.
Things that contribute towards people not having kids (in any order):
Prices are more expensive as a result of having more people (remember that for a long long time population was stable until artificial fertilizer allowed for pop boom) so people cannot afford to live off the father alone or with the mother having a small job only.
corporate culture reduces the window of time where mothers can have children, often they wait until their 30s rather than start at 18 or 20, meaning less than half the time to decide to have a second or third child, people haven't recovered or passed enough time to decide to have more kids.
Housing is expensive, way too expensive in many places to afford to have a family.
(This I have seen and have heard women say this), in many cases (don't know the reason really as I am a guy) women date men and say they love them but don't want to have children with them, then sometimes divorce and decide they wanna have children with someone else, It might be related to men getting overall less "manly" or something, I really don't know how instincts work but may be related, if a guy is (what they call in Japan a grass eater) they might not want to have children with them even though they are in a long term relationship and in turn it might reduce the birth rate (as if they divorce and remarry that happens later on and they might only have time for one). but this last one is pure speculation, it's just a phenomenon that I've seen happen often.
Bad outlook for the future, if things look bleak or seem like they will stay about the same, it might not incentivize people to have children.
Either way, is it so bad? a lot of systems in the west rely on it, yes. but there's also over harvesting, look at the regions with the best dirt in China, they are extremely over harvesting their land and may run into a crisis at some point.
On the other end, population crisis in China is sort of a meme as well, because if you look at statistics 20% of young people are unemployed in China, means that a drop in young people won't really cause a gigantic crash, they are automating a lot of their industries and may continue to increase so.
I think we need a reduction in people to stabilize the world and the resources that we use, I don't think 8 billion or 10 billion as its projected is healthy for the world, we are over consuming natural resources to an extent where nature cannot recover in time