Slight sidetrack:

There is, of course, one DOOM scenario (ok, one other DOOM scenario) which is entirely respectable here-- that the earth will be engulfed when the sun becomes a red giant.

That fate for the planet haunted me when I was a kid. People would say "But that's billions of years in the future" and I'd feel as though they were missing the point. It's possible that a more detailed discussion would have helped....

Recently, I've read that school teachers have a standard answer for kids who are troubled by the red giant scenario [1]-- that people will have found a solution by then.

This seems less intellectually honest than "The human race will be long gone anyway", but not awful. I think the most meticulous answer (aside from "that's the far future and there's nothing to be done about it now") is "that's so far in the future that we don't know whether people will be around, but if they are, they may well find a solution."

[1] I count this as evidence for the Flynn Effect.