So our second derivative is negative. It is fundamentally unsustainable for anything else to be the case forever, right? That would imply linear or greater growth in perpetuity on a finite planet where we and our livestock already comprise 96% of all mammal biomass.

The drop in sperm count is of course concerning, though if we can't get our act together on birth control and family planning then it's probably one of the least tragic ways for our population to get knocked down to a more sustainable level. Humans generally fail at prisoner's dilemma ("if we have fewer babies, other nations will overtake us!"), so it seems necessary that some outside force eventually mitigate our continual failure there--and again, when compared to mass wars or starvation, lower fertility rates is actually a pretty peaceful course-correction.

We largely have gotten our act together on family planning, which is why birth rates are below replacement everywhere but sub-Saharan Africa, and dripping there too. Only reason population is still increasing is a combination of lag (it takes the smaller year groups now growing up in India and China to become adults before those countries will tip into population decline (excluding immigration), and waiting for the decline in Africa to hit replacement as well.

UN projections indicate somewhere around the end of the century for a net reduction in people, but long before that we'll see developed countries starting to compete agressively for immigrants, combined with campaigns to push fertility up to try for a soft landing.