I don't think you can really fault 70s environmentalism for worrying about overpopulation and not predicting the demographic transition considering how little we understand about what's causing low fertility. It's "causally" unpredictable to us right now even as it is happening.

Sure, every exponent eventually becomes a sigmoid and maybe they should have predicted that, but historically the way population exponents turn into sigmoids is via starvation. This one is new.