For the vast majority of people, S3 means cheap, mass storage.
For the majority of those people, a simple SFTP/rsync/NFS/whatever endpoint (potentially with replication to provide redundancy) would more than fit the bill, and would actually be simpler to use than the S3 API.
I store about a terabyte on S3. It costs us $25 a month. If I spend more than a few minutes a month administering an alternative setup, I've lost money compared to just paying S3.
That entirely depends on how you use the data. If you store files that are accessed over the general internet, and each file is downloaded just once in the month, you've just quadrupled your monthly cost, thanks to AWS' high data transfer fees.
For the majority of those people, a simple SFTP/rsync/NFS/whatever endpoint (potentially with replication to provide redundancy) would more than fit the bill, and would actually be simpler to use than the S3 API.