XFS: the filesystem of the future?

It doesn't need to be

> writing the metadata to disk may not be atomic.

That's why you use a journal

> If your writes to disk can't be atomic, how can the entire transaction?

Heard of TCP? It creates a reliable connection over an unreliable network.
Or databases? Atomic transactions on unatomic disks are very possible.

> some of the data you write to a file may be visible before the metadata gets changed, which would make the change overall not be atomic.

No, because you write the new data to *new* blocks. Blocks not references by any file yet.