And they are now some (if not the) largest communities on those subjects on the Threadiverse (and the top communities on their instance), so I consider that point demonstrated.
It's like the original idea behind the Net - if a node goes down, everything shifts to accommodate. If the power posters switch to a specific community people will just use that more.
Reasons for starting a new community:
Avoid centralisation
Avoid tankie instances
If you don't agree with the current Mods or Admins
The problem is that the community you might want to be involved with might be under them. The mods do not represent a community, and may even work against it. This is sort of what happened in reddit's worldnews, where its mods have worked to make it be the neozionist hotspot it is today.
You end up doing pretty much what you do in reddit if you want to continue participating in a community, just creating another alt and try to post when the asshole mods are not active. Most people don't just move on to the discount alternative community with 90% less people participating in it.
The whole algorithm of what a community is really needs to be reworked, but most people on lemmy just seem to want discount reddit.