[themacguffinman]

On the contrary, I think you don't jump from nothing to tangentially-related communities. The reason why niche community subreddits are so successful is because reddit itself is a global, central network that congregates users all on its own with its popular default subs. You have it the wrong way round: niche communities benefit from a global platform that funnels users to them, niche subreddits do not drive the success of the global platform in the first place (although it may enhance it).

Discord and Mastodon could never be as successful as subreddits for this reason. It's pretty difficult to establish a niche community by setting up shop next to other niche communities, niches are full of passionate people who don't spread their passion across niches thinly. You want a global platform like reddit to expose your community to the masses to unlock the niches within it.