You consider PostgreSQL more scalable than dynamodb ? This is certainly a novel take. Since dynamo is hosted are you talking about a hosted pg as well or how even are we comparing this?
And even with Postgres, I doubt their definition is “multi AZ with a shared storage layer between data centers that is also fault tolerant and replicated across multiple data centers”. Meaning that if your database server goes down, your backup servers are immediately up to date because of the shared storage engine in the same region but the underlying storage was also replicated across multiple AZs.
Not if you need multiple databases for failover or read replicas. Aurora’s storage engine is head and shoulders above what you are going to get for $10