I don't think religion is ever going away. I do hope it will become a lot more progressive, as the more extreme versions of religion are what divide us to a great extend. I could easily be a UU, if there was a UU fellowship in my town. I think I understand, as Joseph Campell called it, "The Power of Myth." I simply don't feel that our brains are rational enough to ever totally reject mythology.
Now, if you are asking atheists what they replace religion with, I think it depends on the atheist. Atheists that were very dependent on their churches for support, purpose and joy, are probably going to want to form their own groups to obtain those same elements. I have never missed having a church, but it's easy to feel isolated as an atheist in the Bible Belt, so I've been a member of the Atlanta Freethought Society, although we rarely attend anymore due to the distance we have to drive. We have a small local group of atheists in my town that get together at least monthly for dinner and conversation. It's just nice to be around other people that share your world view to some extent. Why do so many people stay on this forum? I would imagine that part of the reason is to be able to discuss things with others that share the same views, or to be able to openly disagree with those who don't.
I tend to think that humans are far too tribal to ever be very united. Regardless if we're speaking of religious or political ideology, there were always be some extreme versions that cause harm. The end of religion isn't going to change that, imo.
Having family and a good social support system are what most people need. Even the folks that claim to be loners join groups like this one. There are very few people in the world that are able to live happily in total isolation from others. There is evidence that older adults who are not socially engaged don't live as long as those who do, and are at a higher risk for dementia. So, as an older adult myself, I try to stay socially engaged with others, in addition to my wonderful husband.