This was simpler for the initial release. We may be able to offer more granular options but the issue is that the backups belong to the website and not the server that they are on; if the server changes you will want the backups to remain where they are.
Powie Do we still need a dedicated backup server with btfs storage or is it possible to use ONLY s3 ?
You can run only S3 however Enhance backups are a lot more space efficient and faster to create and restore. S3 backups are mainly intended for customers with smaller clusters who do not want to have a dedicated backup server or VM.