Assimilation speed versus rate of joining versus rate of people leaving all need to be kept in balance to have a long running, tight-knit community. An interesting example is university clubs: they have the intrinsic property that people's tenure in the club is very finite, typically a maximum of 4 years. Despite this constant turnover of membership, clubs tend to maintain a relatively constant culture. I posit that the main driver behind this is deliberate accelerated assimilation.

Right out of the gates, you have selection bias in who wants to be in your club: probably people who share many of the same interests as the club. Then you have further selection pressure with inviting new membership: existing members are more likely to accept into the organization prospective members who are similar to themselves, or at least, an idealized version. Then, you have rituals, ranging from hazing to positive Big/Little type interactions, which proactively push new members to assimilate into the club.

Those are good points; a university club almost by definition has 25% turn over each year, but it's scheduled, defined, and worked around. People don't usually just disappear; they hand off responsibilities and train, either explicitly or implicitly.

It probably boils down to mathematics. If your average interaction is with someone you've never met before, it's very difficult for persistent relations to form.