There are already several such services, the most major being Bitpay. However, with their software as a service model they don't charge a percentage of every transaction, but rather a small recurring fee for using the service. Thus as transaction volume increases costs per transaction decrease.

The major cost for current payment processors (i.e. mastercard, paypal, etc) is that they end up third party mediators in disputes regarding chargebacks. The Bitcoin protocol, which inherently disallows chargebacks, eliminates much of that need, thus the fees decrease. What you're paying for with Bitpay or Coinbase is a piece of software that interacts with the Bitcoin protocol for you, not a traditional payment processor that ends up mediating disputes.