So, it’s a fairly common sci-fi trope that we’re going to destroy the Earth via over-population or at least need to do things to deal with (Ender’s Game, Niven’s Future History stories, The Moon is a Harsh Mistress, myriad others). Otherwise, there is some disaster that causes a population drop (Children of Men, Handmaid’s Tale, others).
But now it is looking more likely that we’re just going to voluntarily stop having enough babies to maintain the population (here is an article on India(!) having below replacement fertility).
The beginning of the story is a largely post-scarcity Earth, where the freedom to pursue your own desires has vastly reduced human population due to the time/effort involved to have and raise children.
That fact is one of the purported casus belli for the events of the rest of the series (avoiding most of the spoilers).
I’m not sure I recommend the series… it, like a lot of Ringo’s works are fraught with some questionable assumptions about sexuality that I’m not fully comfortable with endorsing. But it’s still a pretty good story/setting, and if you’re comfortable with an (IMHO) excessive focus on non- or semi- consensual sex…
There is the argument that the whole history of Sparta as we know it (including the movie 300 ) is in fact a non-apocalyptic population collapse story. The whole thing could be later Roman commentators worrying about Roman young folks not having enough kids and getting soft about the kids they do have.
Charles Stross’ Neptune’s Brood and Saturn’s Children take place in a future where there are no humans, only robots, but there was no sudden disaster, just a gradual decrease in population (possibly because interacting with robots was more appealing than interacting with other humans*)
Its not exactly a ‘collapse’ story, but there have been stories where irrefutable evidence of an afterlife is found, and people start committing mass suicide due to it since they feel the afterlife is better than this life.
I can’t find the names of other stories with this trope, but I know there are stories where an eternal afterlife full of bliss and utopia is proven, and humanity basically collapses since people lose motivation to work and live knowing that life is just a waiting room for eternal bliss in the afterlife.
) is in fact a non-apocalyptic population collapse story. The whole thing could be later Roman commentators worrying about Roman young folks not having enough kids and getting soft about the kids they do have.