I think the killer is that right now we don't need absolute numbers, rather relative numbers of N young people per old person. We are net producers of stuff up

I think the killer is that right now we don't need absolute numbers, rather relative numbers of N young people per old person. We are net producers of stuff up to some age, then become consumers as we retire, and need some producers to offset us. You could say, meh, you saved up money for retirement so you're good, but (a) that's just you, not most people, and (b) even then you need some young people to bake break, fight fires, look after you when ill and so on. I guess we will have to break this model, whether we like it or not, but right now we rely on exponential growth... The longevity, ironically, is making it worse, as there are more old people, and I guess by extension more frail people, who in the olden days wouldn't make it, all of whom need more care from young people.