I'd have to disagree with the premise that the Twitter migration "failed", for a start. Of course with anything like this you get a big spike and then it trails off. But the question isn't "did we retain 100% of people who tried out the new platform?" it's "are there now more people on the platform and are they having a good time?"
I never actually used Twitter (yet somehow had about nine accounts but I kept telling myself this time it would stick). Now I have two active Mastodon accounts with two active feeds and am having a great time.
And the same applies with Kbin/Lemmy. It's not for everyone. Nothing is.
But are there more people here now? ✅
Are we having fun and interesting conversations and a generally good time? ✅
I think niche communities are a result of platform size, and trying to force it too early doesn't really work. What we can do is make sure the slightly-less-niche-but-still-fun communities that already exist are looking nice and active and welcoming ready for whenever there's a new wave of users, and the niche splinter groups will come with time.