It seems to me that the main thing atheist groups do is give people a place to land when leaving religion, and help people in more religious parts of the country connect with like-minded people (see here for some discussion). As a result, they can ironically be stronger in more religious parts of the country.
And it seems like there’s a similar dynamic with a lot of James’ other suggestions as to functions for a “humanist community.” In a culture where religious weddings and funerals are the default option, there’s value in humanist organizations providing resources on how to have a non-religious ceremony. But if we could get to the point where non-religious weddings are normal, non-religious weddings would just be “weddings,” not “humanist weddings.” That’s how it is right now in South Korea. (ETA: Some discussion of current US practices here.)
Similarly, in a country where lots of schools deliver terrible sex ed thanks to religiously-motivated sex-phobia, I can see wanting to take your kids to the Unitarian Universalist sex ed courses (which I confess I know little about, though I’ve heard they’re very good). Even in the current environment, though, I’m not sure “humanist” (and liberal religious) organizations are the best people to count on to develop stuff like that. I’d ask existing, nothing-to-do-with-religion-or-any-specific-philosophy sex-positive educators for advice on that before I’d go to a “humanist” organization.
In other words, the value of humanist organizations comes from the current dominance of religion, not anything inherent in the nature of the things religion currently dominates. And if people can figure out how to do those things without any help from either religion or “humanism,” more power to them.