@Crell @jerry @jztusk (Back of napkin caveat)
Currently seeing one server per 500 accounts. Writ large that's 10M AP servers for 5B accounts.
Several hundred million not-WP.com WordPress sites might offer an example. Value-add hosting options abound for WordPress, so it's partially self-hosted insofar as it's somewhat self-managed by tens of millions.
Mix of managed, shared, and self-hosting for AP services coupled with a rich plugin framework is how I imagine adoption could be supported.