I just exported all of my MariaDB 5 databases and then imported them to MariaDB 10. I then turned off MariaDB 5. When I did so none of my WordPress websites would connect to the newly created MariaDB 10 databases. 

The ONLY way I was able to resolve this was to use Step 4 of this procedure: 

Steps:
1 backup MariaDB5 and MariaDB10 (option in Hyperbackup)
2 export all databases from MariaDB5 to a sql dump (option in PhPmyadmin)
3 import sqldump in MariaDB10 (option in PhPMyadmin)
4 edit wordpress wp-config.php: search for "define('DB_HOST', 'localhost');" and change in define('DB_HOST', 'localhost:/run/mysqld/mysqld10.sock');

Why is this necessary?

If MariaDB 5 is deactivated, shouldn't the Synology NAS automatically default to MariaDB 10?