Before you or someone else mentions Mastodon (and probably compare it to the Reddit migration to Lemmy; I've been through this conversation before): Both Twitter and Mastodon are built on the concept of following people. If those people don't migrate to another platform, then the people following them won't migrate, either.
But Reddit and Lemmy are built on the concept of following ideas. It doesn't matter if one person who, for example, enjoys anime, only stays on Reddit. Others who who enjoy anime may move to Lemmy and become part of one or more of the anime communities on Lemmy instead.
Because there's no viable replacement.
Before you or someone else mentions Mastodon (and probably compare it to the Reddit migration to Lemmy; I've been through this conversation before): Both Twitter and Mastodon are built on the concept of following people. If those people don't migrate to another platform, then the people following them won't migrate, either.
But Reddit and Lemmy are built on the concept of following ideas. It doesn't matter if one person who, for example, enjoys anime, only stays on Reddit. Others who who enjoy anime may move to Lemmy and become part of one or more of the anime communities on Lemmy instead.
Basically, the comparison isn't 1-to-1.