It doesn't really fit the picture. Capitalism is about 250 years old and most of that time it correlated with a massive population explosion, not a collapse. The current world also isn't uniformly capitalist. Socioeconomic conditions and systems differ across the globe, but the collapse seems to be nearly universal.
There are more things at play. For example, we spend our most fertile years in school and we mostly eliminated teenage pregnancies in the developed world. Which is likely better for the socioeconomic level of the now-not-mothers, but it also has negative biological effects and sank the overall birth rate in a non-trivial way.
What is entirely new is the loneliness epidemic, though. I would blame that on the specific combination of Covid lockdowns (which killed off a lot of real-world institutions where people met in person) and the smartphone attention economy. That is a very small subset of "capitalism", though.