Postgresql is the better product, but doesn't have the horizontal scaling of MySQL/Maria though, so if you want an easy to setup cluster MySQL for high volume online retail store or similar has a use case still.
i.e. MySQL/Maria's MM is one of the worse options compared to anything else.. like TiDB or Aurora on AWS for MySQL compat and Yugabyte or CockroachDB for Postgres compat (or Aurora on AWS or AlloyDB on GCP).
Usually there will be one or two tables that grow at a dramatically faster rate than everything else and I have always found that those belong in a separate data store.
Horizontal scaling for DB typically means distributing the write load across instances either via sharding or master-master. I could be wrong of course, but that’s how I read it.
MySQL is very good at master-master out of the box.