Well, I don't think I've ever argued in favour of that - Carter just assumes it. To make my position clear, here, I've simply been trying to explain why it's rather harder to make Carter go away than some folk imagine - he's been vexatiously discussed for 20 years, after all, and that's not a common characteristic for an easily-dismissed logical error.

So you're saying that information external to Carter's assumption may constrain the lifetime of humanity in some way. By analogy, we might imagine a community of human liver cells trying to use Carter's reasoning to predict their community's future existence. Cells in a 7-year-old child would predict a relatively short future, while cells in a centenarian would predict a long future. Oops: the information they lack is that there is a characteristic lifespan for a human.
However, to use this argument to properly blow Carter away, you'd have to tell us which specific bit of information you have that gives you a more reliable estimate of humanity's future existence than Carter does.

Grant Hutchison