Add-ons that store custom data and need to migrate it are probably in the minority.
Some may do that, but they also need to balance it with their users complaining about a WebExtensions version, and the add-on potentially breaking because of the compatibility changes happening in 55 and 56. There’s a statistics dashboard where developers can see the popularity of different versions, which should give them a good idea of user migration.
We have a very good idea of who is or isn’t migrating, at least among the top add-ons. Since these lists are hand-curated, there’s only so much we can do for the long tail.