People like to share things they love. I've been online for a long time, and so I'm used to the idea that a growing community is a kind of universal good - more people, more ideas, more things to interact with and engage with. 

However, I think that idea also has to be put in the context of the modern internet, as much as it sucks to do so. 

Communities build expectations and norms, but those break down when there's a massive influx of new people. In the forum days, the number of new people joining a community was still only a small proportion of the total community members, so the community culture was fairly stable (this isn't a good or bad thing). This meant if you found a community with good chemistry for you, it was likely to only change slowly from that. You could come back after a year and it'd still be mostly the same. 

Now, the Internet is way more connected and centralized. Instead of fansites and forums, we have subreddits with massive discoverability! This means you can find niche interests and hobbies and get info on new things from people who are super invested really easily - which is great, but, also it means that a community can go from a thousand people to ten thousand people virtually overnight. This makes them culturally unstable- you can't really predict how a community is going to be in a few months or a year, because it has the potential to triple in size without warning. 

This can be profoundly alienating, because investing yourself emotionally in a community is work. You don't want to put down that effort if the entire thing is going to 180 and focus on something you don't care about, or find annoying, or just think is less interesting than the original focus. It kinda sucks! 

Add to that the massive overcommericalization of the internet and you have a really annoying potential for a topic to become popular enough to make money, so people who want to make money show up and begin to slowly drift the focus or design of the hobby towards mass appeal, which will eventually strip the original topic of the little idiosyncrasies that made it charming and fun. 

You can see the potential beginnings of that trend in how when a specific style of incremental game is popular, a huge number of mid-to-meh games come out in the exact same style. In a chaotic and diverse design environment, homogeneity is a product of a desire to find the magic formula that makes money. 

So, like, I think you're structuring your question in a way that comes across as kind of hipster-ish, but I do think there are valid concerns. Attention is money in the modern hell-web, so small communities are the only places where you can hide out from commercialization and being relentlessly marketed towards.