It's not that it is unplanned, but that it is a big job that cannot be done all at once and, as such, the plan is a long term one.
The MySQLi extension is the first step that was needed to make the migration, the other steps will follow.
As the other extensions are upgraded to offer MySQLi support, they will still be backwards compatible with previous pages that are developed using MySQL.
Rome was not built in a day, neither was the migration to MySQLi.
We have not made any final determination on what features will be included in the MySQLi upgrades to the other extensions, or any upgrade pricing.