I think "intelligent" is perhaps the wrong adjective to distinguish humans. I would suggest "social" is a better one, as we are to other primates what ants or bees are to nonsocial insects. It's not the intelligence of the individual so much as the "programmability". Thus, I doubt it's an evolutionary dead end, as when you look at social organization from a broader view, it's developed in many different species. Even multicellular life itself seems like social organization at a lower, earlier level of abstraction.

I would agree that an interplanetary/interstellar civilization is something that may never come to pass, but I don't see it as a judgement on the capacity of particular life on earth to adapt, but rather a result of the physical laws that constrain all life, particularly delta-V and the speed of light.