That has the potential to cut both ways. If Amazon decides that S3 is their loss leader, then they would absolutely bury boutique players who only cover a few functional areas.
If instead they decide it's the cash cow, then it wouldn't be a stretch to predict that they price it a bit above the competition. The customer has the impression that they can amortize the cost across the other value-adds in the ecosystem. Whether the customer is right or not barely matters to Amazon from a purely financial aspect. All that matters is that they believe it to be true.
There's also the short-term versus the long term strategy. Short term they could do either and I wouldn't be surprised. Long term, I think I would expect the latter.