Getting retailers on-board is definitely core to the success of Relay. As announced today Saks and the SAP Hybris platform are live on Relay and we're working with numerous other retailers as we speak.
For apps, getting retailers to sell on their platform is a huge pain (custom integration to their APIs and payment systems) convincing them to do so. For retailers as well it's a pain. They have to create technical integrations with as many channel out there. That's why we think Relay make sense. We help both side of the equation.
Also, the relationship between a retailer and a channel app in Relay is opt-in today. This is literally a Stripe Connect (think OAuth) connection. But once a channel is onboard, it's nothing more than clicking a button for a Relay retailer to start selling there.
Also I wouldn't say they are losing control on the full shopping experience, they're merely enabling a better one on mobile. When you think of the shopping experience, a lot happen after the purchase (support, shipping, loyalty) as well and I think there are new experiences to create there, especially once the customer receive the receipt from the retailer.