A) Postgres easily scales to billions of rows without breaking a sweat. After that shard. It’s definitely negotiable.
Statements like yours are meaningless when you aren't specific about the operations, schema, access patterns etc.
If you have a single server, relational use case then PostgreSQL is great. But like all technology it's not great at everything.
In all seriousness, calling Postgres’ scalability “not-negotiable for most use cases” is wild.
"built-in, supported, proven scalability and high availability"
PostgreSQL does not have any of this. It's only good for a single server instance which isn't really enough in a cloud world where instances are largely ephemeral.