It feels like the obvious first step is for the people with many followers to start posting all their content on multiple sites, so that the second point no longer applies.

The trickiest part is the lack of a fully agreed upon alternative. Do you go to Mastodon, instagram, threads, etc? While some folks might have an alternative that they view as a no brainer, it's pretty clear that there isn't overwhelming consensus. Mastodon seems perhaps the most popular as a direct Twitter alternative, but it regularly faces complaints about usability and discoverability. If people post to both Twitter and some random alternative, if that alternative varies wildly, followers will stay on Twitter cause it's where all their favourites are.