Not a substitute at all. It would be like getting rid of football, but still wanting people to go to stadiums for the sake of going to stadiums. Football was the reason they went.
The religions bind mostly due to the doctrine of the religion, not the activities of it. UU has no real doctrines and no binding point. Without that doctrine motivating you to do the activities, people just stop or don't bother. This is why secular attempts will mostly fail, because the doctrines they do have tend to be extremely narrow and non-universal, i.e. knowledge worker-style atheism.
That hasn't been my experience in the Episcopal Church. We're pretty liberal and don't do a lot of telling people what to do. People may come for the doctrine, but they stay for the ceremony and the fellowship.
That's my perception of church-based community. The doctrine is really just an excuse to get together and form community bonds. By your analogy, I think it's more like switching to, say, track and field, not gathering in an empty football stadium--a very different sport, but still a reason to get together and cheer. I'm not sure how that can be translated to the secular world, but it seems like there should be a way.
(I think a lot of modern atheists don't realize that liberal Christians exist. The awful fundamentalists dominate the news cycle so much. Most of us are happy to mind our own business, do charity work in the community, etc., but none of that makes headlines.)
The religions bind mostly due to the doctrine of the religion, not the activities of it. UU has no real doctrines and no binding point. Without that doctrine motivating you to do the activities, people just stop or don't bother. This is why secular attempts will mostly fail, because the doctrines they do have tend to be extremely narrow and non-universal, i.e. knowledge worker-style atheism.