Your sarcastic point is kind of valid, IMHO.

Humans establishing self-sufficient communities beyond Earth would prove that humans have the capability to thrive sustainably on Earth. It would silence forever any technological objections to the possibility of human sustainability on Earth.

And a firm idea that humans can never establish self-sustaining communities in space is not that far from the idea that we probably cannot do so on Earth, either.

It really is not a huge step from pessimism about human future (ie in space) to nihilism about sustainability on Earth, and I do wish people would acknowledge this more: people enthusiastic about permanent settlements in space are, in fact, more certain about the possibility of sustainable thriving on Earth than the space-pessimists are.