> It'll be a local max, not a global max.

There really isn’t any way to know this for a fact. The future could hold technology that allows us to expand far beyond the current population, but it also could lead to setbacks that the population never recovers from. It is reasonable to guess it’s a local max.

I think this argument would make more sense if it were external constraints that were driving a declining population. But the population is only decreasing because the majority group of people stopped having children. So they will remove themselves from the gene pool, the minority will become the majority, and away we'll go again.

As an interesting factoid the Roman Empire, which for many people of the time would have had some analogs to 'the world', also had a fertility collapse prior to its end, that they tried to combat with quite strict laws, but ones which were ultimately ineffectual. Of course that was hardly the end of the story!