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For example, pople may believe (correctly or incorrectly) that having two children who expect to live to 40 is not necessarily better than having one child who expects to live to 80.

I think the problem is that someone supports you until you're about 20 or so, if you work 20 years and live until 40, then no one will have to support you after you die. But if you reture at 60 and then live until 80, you have to be supported for 20 more years and are a lot more expensive than you were those first 20 years. So for this it's better to have 2 who'll cost society anything, but having 2 who'll work until 80 would be even better. If we're going to live long, then we just have to get used to the idea of working long.

53 - You bring up a very interesting point, but you come to the wrong conclusion - "I think the problem is that someone supports you until you're about 20 or so, if you work 20 years and live until 40, then no one will have to support you after you die"

Very good - you are a break even, if you live to 40. But if you live to 60, then you can support an extra 20 year old. And if you live and work till 70, then you support an extra 1.5. And if you live to 80, and still retire at 70, (the new retirement age) you still support an extra 1 up to 20 year old.