I personally want to have an SQLite as my default database, as I have a few friends' websites that have extremely low traffic. There's no need to have MySQL installed for such a small website that contains basic information.
No, SQLite can handle high volume of traffic as well and those who don't believe me, go check https://levels.io/remote-ok/
But that is not the point here.
We really need to have more alternatives to MySQL / MariaDB / Persona.
Thanks to PDO, we could have MS-Server, PostgreSQL, SQLite, Oracle...but we really need to support only the three most used: SQLite, MySQL, and PostgreSQL.