Not even just the niche communities, there are plenty of large communities that just don’t have a presence here whatsoever. Show subreddits, for example, don’t really seem to exist or get any attention, apart from Star Trek and Futurama. If I want to talk about the new episodes of Invincible or Rick and Morty or Doctor Who, Reddit is currently the only place, unless I want to sit around talking to myself in an empty Lemmy community.
Conversely, though, filling dead subs with mirrors of reddit content that we can’t seriously interact with feels like it’s just ramming home the point that the specific Lemmy community in question is dead. It’s like saying “come to Lemmy where you watch reddit have a discussion, but not join it.”
Ultimately this whole idea is trying to apply a tech solution to a human problem. If people won’t come here and fill the place with content themselves, that’s not an issue that can be programmed away without Reddit’s API access, but if we had that, we wouldn’t be here in the first place.