I've seen some communities thrive for decades. Their success seems to come down to a few ideas:

Focus on the community itself, not on the people outside it. Underground is always defined relative to the mainstream, which isn't healthy for the community.

Don't give the original members any special status. They either fit in, or they move on.

The community must reinvent itself once in a while. The world moves on, and the original concept will eventually be obsolete.

Keep the key institutions non-commercial and under the control of the actual community. People trying to make money from the community are not a threat. Turning the community into someone's business is.