This in turn makes joining the community less attractive.
That sounds like it just hits the natural limits of a community? Doesn't sound too bad.
It's not too bad, unless new (younger) members are your primary source of wealth and labor. A commune primarily consisting of retirement aged and older adults will be far less productive, and will eventually reach a tipping point of unsustainability.
This definitely plays a part as far as I can tell. The two communities that I mentioned that are looking most sustainable have an important thing in common - they both operate large businesses (orchards and gardens). Many of the others have devolved to the point where members live on the land but seek work elsewhere.