It's a problem we chose to create for ourselves by inventing and socially normalizing 99% effective birth control. We've given ourselves the freedom to make the choice to have children consciously, but not without consequences, some of which have already caught up to us.

In the past, natural selection had no reason to distinguish between the drive to have sex and the drive to have children because they have the same effect. If we abruptly take away the former as a reliable enough mechanism to propagate the species, we should expect much fewer children. And that is exactly what happened when the pill was introduced a few decades ago: it cut the birth rate in half in developed countries almost instantly.

If we wanted to ameliorate the demographic issues caused by this, we just have to restrict the supply and availability of birth control pills and similar surefire methods like IUDs, just enough such that the birth rate is pegged at 2.1 children per woman.

But this is difficult to achieve in democratic societies because the people that want sex without reproduction make up a significant portion of the population. But we have to fill the labor gap somehow. The solutions we've come up with are mass immigration, and soon, AI. Of course, these solutions create new problems that we'll have to deal with later, but such is life.