but the millions of small niches are lacking still
Even Reddit didn't fully solve that problem. When you go outside of the general topics it an be surprising how slow activity can be. That's simply the nature of the niche.
There's no real way to solve that without simply being that change yourself. Because those niches tend to have the 2-3 same users posting half or more of the discussions or links.
they aren't wrong. It will massively deflate their community. That's an ineivtability of how lurkers on the internet work. They aren't there for community, they are there for easy passive browsing.
"we" as in the common person? It won't be a fast track. There will need to be a steady supply of content for a certain topic, and a stream of discussion. Unfortunately the best way to help as a single person is to basically become that sweaty forever online person. The first step to the Network Effect is to generate enough content to engage with.
If "we" have developers or artists that can be one bigger step to help out. contribute to making apps and extensions to either bridge the gap or overcome current shortcomings of these federated instances. Even amongst techy communities there is a lot of confusion to how instances work. So some app to make it dead simple to browse and comment (while later allowing options for power users) is key. Sync committing to working with Lemmy/KBin is definietly a bit help.
Most of the rest is up to the instance admins. SEO, improving features, getting good moderatiors, etc. None of that is in out control, we can only give feedback