Pareto principle applied to human progress. Most progress comes from a small subset of the population. The larger the population, the more of these outliers we have and the more progress we have. Ergo, depopulating the planet would have a chilling effect on progress and may even lead to civilization imploding and sending us back to the stone age.

I'm 1000x more concerned about running low on talented people than I am about us running low on commodities/energy/land.