Well, I've been reading on The Economist how China is seriously struggling with youth unemployment right now. Lots of fresh university graduates are sitting unemployed, or doing manual labor. In my home country (Finland) we have had way below replacement birth rates for a long while now, yet youth unemployment is at all time high.
The problem with low birth rates is assumed to be that there are less workers to take care of the pensioners. But right now it seems like even the few young workers available often aren't needed anymore, thanks to increasing automation of industries. I predict youth unemployment will be made even worse by AI soon enough. So, why do we need children, if capitalist economies don't need workers? Unemployed people do not pay much taxes to sustain the non-working age population.
I believe low birth rates might become a blessing soon enough. It just means less people for governments to take care of when robots and AI take away most jobs.