I think there are many strong arguments against this, but most are debatable. So I'm going to focus on one of the arguments that I do not feel especially strongly about, but one that I think is simply undeniable.

A smaller world population in the longrun is a defacto nonstarter. Many places in the world continue to have extremely high birth rates. And the wealth/education correlation that claims to predict the change of this seems, at best, extremely weak when put under scrutiny. Billionaires have high wealth, high education, and high fertility. Places like Thailand have extremely low education/wealth levels, yet their fertility rate is lower than even the US. Israel is the third most educated country in the world, has an advanced economy, and a fertility rate that is even competitive against some African nations at 3.0.

The issue is one of prioritizing wealth more than family, which also explains why the trend collapses at billionaires (for whom family has effectively 0 impact on their wealth), even though the correlation would expect them to be having effectively 0 children. And this pursuit of wealth over family holds true in the West, but there's no reason to expect such ideology to spread. So the question is not one of a smaller world population, but of a smaller population of the groups that are not reproducing vs those that are. In other words should the West simply accept its own extinction, likely alongside those of its values? And to me, the answer to that question is no.