I'd wager that those communities that so called "get ruined" didn't grow as their size grew. What I mean is that the rules, norms and the culture that allowed X number of people to thrive in a niche, most often does not translate to much larger Y size. If a community decides to allow growth, it must also be flexible to adapt to that growth. And let's be crystal clear on one thing: communities grow because they want to grow. There exist sites online that have very strict sign-up policies that ensures they are kept small and/or limited to a certain type of users. Some even show long questionnaire on their sign-up forms and you have to wait for sometime to contribute and have to stay active to keep your membership, etc.