There are many things that people at age 10 can't imagine about people at age 20, or 20 imagining 40. There's no particular reason to believe that problem goes away.
That said, I'm pretty sure that's the same sort of argument I'm suspicious of. It's not an honest question that "will you"; it's an argument that the answer to that must be no. Not because of any evidence that has been presented, but because you can't think of any answer.
And I think it works because he's assuming an answer to the question he's purportedly asking. We don't really understand what minds are or why bodies age. We don't even understand exactly what happiness is. And he's presuming that a choice like that is possible.
What we do know is that a choice like that is currently unavailable, and I think it's dangerous to build too many sky castles on the assumption that it is.