> Only if the mothers in aggregate truly believe that their children will have good lives, then will they have them.

Parents have never truly cared whether or not their children will have "good lives", certainly not in any - "i'll sit down and analyze carefully if my offspring will have a good time" type of way.

Child mortality rate used to be something like 50% in past.

People still have insane fertility rates in complete - objectively shitholes - like Bangladesh, etc.

That's simply not how the world works, that's not how natural selection works.

The problem is that you (and most people frankly) look at the "fertility problem" within their very limited 1-human lifespan. However, if you zoom out a bit, the fertility problem disappears, not only does it disappear completely - the problem will disappear regardless if circumstances get better or if they get way worse.

The mothers (and fathers) that don't have children because they think the "world as it is right now is a bad place", will simply get selected out.

Caring about whether your children will have "a good life" to a point of not having any is simply maladaptive from natural selection POV and it will sort it out very quickly. It's just a 1-gen outlier.