Well for now I'm used to have both...
At office they were using both Oracle for critical software and Mysql for lower-end software, now that MySQL is owned by Oracle, the support is "mostly" included in the horribly expensive license for Oracle Enterprise
(something like silver or gold support, I don't know the details exactly...) so they haven't switch to MariaDB.
For my part I don't have any more "MySQL binaries", everything has been drop-in replaced by MariaDB, maybe it's still embedded in the "Wamp Setup" I use for portability when working on DOL out of home