You aren't getting anything resembling Tarsnap's security from S3 or GCP. If you don't care about security (and for some data sets, you totally don't!), you're right, it doesn't make sense to use Tarsnap.
I'm not rsyncing plaintext data to S3/GCP, I'm using BorgBackup to encrypt and dedupe it. I don't know if the encryption is comparable between the two (they both appear to use AES-256 and HMAC-SHA256), but I imagine tarsnap's is slightly more clever?
