The problem is that's not a likely hypothesis. There's no evidence lack of capacity is the cause of declining birth rates. In fact there's strong evidence for the opposite: countries with more resources to go around tend to have lower birth rates than countries with less resources. https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?most_rec... There's no equilibrium here, if anything the feedback loop is positive rather than negative. That's the concerning part.

I thought this was common knowledge, but given the reaction I'm getting in this thread I guess it's not as common as I thought and I should have explained myself better in my original comment.

I mean "capacity" isn't some magic barrier. Usually it involves increased chances for the organism in question to starve to death due to reduced availability of food. We don't seem to be that close to reaching that.