But you can accomplish the same thing by creating new more moderated subs. That's why we have /r/AskHistorians in addition to /r/history. If the sub isn't specific enough the solution shouldn't be to falsly and silently fragment the sub's users via user-side filtering, it's to explicitly split off a different community that behaves the way you want. In that way there is no "non too specific sub".

If you participate in a subreddit, but filter out 70% of the content, in what way are you participating?

Sure, but things are wavey and blurry, you can't always foresee if a dedicated sub should be created, sometimes it's in waves.

True, but if your users are filtering out parts of the conversation you'll never reach a point where it's obvious there should be another community created, you'll just have multiple communities talking past each other.