The MySQLi Extension is the groundwork that was needed to be laid for starting the MySQLi migration.
Think of it this way:
For the existing extensions, The DW Server behaviors where the groundwork they where built on. Adobe has depreciated the server behaviors, so we first needed to recreate that ground work which is the MySQLi extension.
Now that the MySQLi extension is released, we will be starting to work on migrating Data Bridge, Design Extender and eCart.
Data Bridge is the first one we will start on. as it is the most database driven of the extensions it will take the most time to complete.