The earth cannot go above carrying capacity. No closed environment can allow a population sustained by it to go above it's ability to sustain, this is impossible. Imagine a fenced field with rabbits, how can the total rabbit biomass ever go above the total biomass of the system? Not possible. What you'd see if the earth were close to its carrying capacity for humans is a rise in infant mortality to keep it at or below carrying capacity. We are not seeing this, therefore we can deduce that we are not near carrying capacity.
The problem that is causing reduction in fertility, if I have to name one that is the biggest cause, since complex things like this have more than one cause, I believe is urbanization. Cities have always been population sinks, they've always relied on immigration from rural areas to sustain their populations, even as far back as the bronze age. We happen to live in a time when more than half the worlds populations live in an urban environment.
The problem with population contraction is that the median age goes up. A world with a shrinking population isn't just a world with less people, it's a world with increasingly elderly people. The ponzi schemes will collapse, and those that will suffer the most are the elderly. You can't have a system where 1 person has to care for themselves and more than 1 parent on top of having kids, it's not possible. It becomes a spiral.
And if you think you can accustom yourself... Imagine even in a non authoritarian environment, stepping over starving people and dead bodies as you go about your business. Having to look a little unhealthy to avoid being robbed. In authoritarian spaces, potentially the execution of elderly, the change in governance structure from democratic to otherwise because of that "necessity" given that elderly are the strongest voting base in all countries that have this problem and it will not happen without the crumbling of Democratic institutions.
He who reproduces at above replacement rate, especially in times like this, will inherit the world, not just genetically, but someone who, in these times, teaches his family a tradition of larger than average families, his descendants will be kings.
