> If anything backups are the key to high availability.
Not really. Backups are complementary in disaster recovery. They play no role in high availability. Putting your data in cold storage plays no role in keeping your system up and handling traffic.
> Streaming replication lets you spin up new nodes (...)
You seem to be confused. Replication and backups are two entirely separate things. Replication is used to preserve consistency across a distributed system and improve fault tolerance, whereas backups just means you are able to recover the state of your system at each checkpoint. Either you're using a word while giving it a new personal meaning, or you're confusing concepts.