[the_reformation]

S3 (and most AWS services) are extremely price elastic; i.e., the lower you make them cost, the more people use it (a la electricity.) That's why they've done stuff like drop from 50ms billing to 1ms billing, etc.

[ece]

They could still offer it as a client library feature, just tell the users what kind of r/w amplification and guarantees they can expect, and it's something they could optimize later or not.

[CodesInChaos]

You can't reliably implement conditional PUTs in the client, you need a server side mechanism like the `if-match` header.