Basically yes. Most MySQL core developers moved to MariaDB, and they added the features that MySQL corporate version has and a lot others on top, for free. They made a foundation and a license that guarantees it will be public all times, so what happened to the product when it was bought by Oracle would not happen to Maria, even if the Foundation is bought.
Postgres has the same downsides and limitations as MySQL and a worse license and dev policy from what I heard (its owned by one company and keep a lot stuff secret and take arbitrary product decisions).
Basically yes. Most MySQL core developers moved to MariaDB, and they added the features that MySQL corporate version has and a lot others on top, for free. They made a foundation and a license that guarantees it will be public all times, so what happened to the product when it was bought by Oracle would not happen to Maria, even if the Foundation is bought.
As for the reasons why you should not use MySQL anymore here are a couple, including that many Linux distros dropped the support for MySQL and moved to Maria.
seravo.fi/2015/10-reasons-to-migra...
Postgres has the same downsides and limitations as MySQL and a worse license and dev policy from what I heard (its owned by one company and keep a lot stuff secret and take arbitrary product decisions).
blog.panoply.io/postgresql-vs-mariadb
Thank you @bgadrian for the detailed response and references.