I suspect a crashing economy is worse for climate change all things considered.

^^^ This. A declining population is good for humanity in terms of reducing climate change (among other environmental impacts), addressing resource scarcity, preventing war/disease/famine, and encouraging states to invest more in their people (which benefits democracy/freedom).

I think the reasons depopulation is generally depicted as a bad thing are: (1) if it happens too quickly then your society doesn't have enough young people working to support the old, (2) capitalism--the most effective economic engine we know of--works best with a growing population, and (3) states derive geopolitical advantage from having large populations.