One really needs to be realistic when thinking about these sorts of things. The classic argument you hear in many walks of life is "I can put together x, y and z to make it do stuff, and it's a very good and cheap solution for a xxxxx that measures better/performs better/whatever than a..."
If the piecemeal solutions were that good and that cheap, then there wouldn't be a market for integrated solutions. But realistically, many piecemeal solutions are neither very good, very cheap, nor effective for sizeable numbers of people. The iPhone plus headphone adapter MIGHT measure well, but it is not designed to function as a dedicated streaming appliance. It doesn't have the feature set, nor is it intended to be used in that fashion. Can you make it work for some limited applications? Sure. But don't kid yourself that you're gonna replace a Bluesound, Sonos, Cambridge or anything like that with an iPhone. They aren't the same.