They aren't starting from scratch. Migrating your database is a complex and risky project, and not the kind of thing you do unless you have a very clear, very pressing reason, like horrible performance and stability issues, or saving tens of millions of dollars in licenses. Migrating to MariaDB is more like upgrading a minor version, with minimal risk and zero downtime. And they have a lot of experience in MySQL, almost all of it transferable to MariaDB.