That's not how subreddits work, though, since they require a a name and almost always have mods that shape the content. /r/waterniggas at 300 people is basically the same at 20,000 people. So if the rule is subreddits should not have "nigga" in the title, then all subreddits with that name should be quarantined (or not allow the names to contain that word).
Subreddits don't tend to change topics or "voices" but they can and do worsen in quality. Instead of posts with original concept C being upvoted, posts with content similar to C but not really get upvoted by the more general audience that comes with larger subscribers. I think /r/bonehurtingjuice is a pretty good example. At first it was excellent but now basically any anti-joke (which is different than bone hurting juice jokes) gets upvoted there.
It also sounds reasonable to me. At small scales, especially with novices, it's good to let people experiment, find their voices, build the right audience. But once something's of a certain size, it's unlikely to change its character much.