I disagree with the premise.
"Is human space flight worthwhile" is really the question you appear to be trying to answer. Calling it reasonable while using a highly individualistic definition in your axiom makes the whole discussion strange.
I mean, suppose The world would end in exactly 1000 years if we do not each invest $10 in preventing it but it must be done in the next 10 years, my lifetime. There's no tangible benefit to me of preventing the world from ending in 1000 years, so I'd be tangibly better off if I were allowed to spend that $10 on a couple of mochas.
But from a species and society level, $10 per person to prevent the ending of the entire world seems pretty obviously a good idea.
The benefits of human space flight may not be tangible, but I think it's definitely a worthwhile thing to do. The benefits go beyond the purely tangible and purely individual level.
So on the one hand, from a short-term, individual, purely economic standpoint human space flight isn't reasonable at all, from a societal and species standpoint (and an individual, what-gets-you-up-in-the-morning perspective), human space flight is eminently reasonable. The opposite would seem suicidal on a billion-year timescale.
Again, it's like the decision to have children. Individually, economically, there's nothing at all reasonable about having children. But species-level it is absolutely essential. And individually, it brings meaning in life.