I think that's actually less contradictory than it sounds. In one case you're doomed in the way where too big a population uses too many resources leading to a crash. In the other case what you're actually doomed by is a historical population boom that requires a large younger population under it to sustain it, and in the absence of new young people, maybe that older generation cannot be sustained, potentially leading to catastrophe.

In fact I'm pretty sure there's a potential middle ground with large enough historical population boom where both of these issues could hypothetically be realised simultaneously...

But, you know I think we should really focus on just making agriculture and energy production more efficient/cleaner, reducing waste and making care work a decent living or something.

Nobody said that threading the needle was easy.