The Copernican Principle seems to imply that all observers see the same kind of universe, namely, an uninhabited one apart from the observers' home planet. The SETI cultists' refusal to accept this observation and their increasingly convoluted conjectures for why we can't detect ET's show that the quest has become irrational by scientific skeptics' own standards.

What does this second sentence mean? If everyone sees that the universe is uninhabitated this makes the Great Filter more of a problem, not less of one, and it needs a resolution. That you don't like some of the explanations doesn't make the attempt to understand what is going on irrational.