I do have to say, I've never understood the European/Western liberal (in the broad classical sense, not the "social democratic sense", but more strongly among social democrats) impulse to devalue one's own culture and values so very much...

I think you are confusing correlation for causation. I don't think the sustainability movement is largely responsible for declining birth rates, but rather that Western culture values many other things OVER child rearing, andmore advanced civilization requires delaying child birth until later. Most of the adult couples I know who are childless aren't childless for ethical reasons, but instead for things like careers,etc. This isn't a devaluing of culture, its the expression of it.

Hence, France managed to bring back their declining birth rates by making it easier to have kids, so the trade-off between (for instance) career/family is lessened. I'd be happy to see other first world countries address the problem in similar ways.