You've described what Reddit went through. I think that's basically the process of maturation of any social platform. Reddit attracts (I.e. automatically subscribes) users to the default subs and so those are the biggest mainstream areas. Most people who want something more (and how I have used reddit for many years) move to smaller subs and find a much better environment there. Or communities break out to make smaller versions (true gaming, true movies, true fitness, etc).

Basically I don't think any of this is platform specific, it's just how things evolve online. Certain features judge development in a certain direction, but in that regard Lemmy should be much better without bullshit algorithms and disguised advertising posts..... For now.