How do you even get the petabytes of data into S3 without incurring massive charges? Previous estimations of getting the initial dataset into cloud was.. prohibitive (note this was 3-4 years ago).
The ENCODE Project data were a hundred terabytes or so at the point I migrated it into S3 (it's now over a petabyte.) We were on a fast academic network with a 100GB interconnect so could just `aws s3 sync` rather than worry about using AWS Snowballs or anything.