For sure we will be at the one device point very soon, if we aren't already there now really. With most headphones designed for mobile use, a phone like the V20 for instance would be quite adequate. I expect (and hope) that DAPs are still always a viable if niche product to satisfy those who demand that last amount of sound quality possible.
Oh for sure.. i am certainly not saying to get rid of them. Specalty built products will no doubt eek out that last bit of sonic quality compared to something like my v20, and should be available as a niche product. More, i am just happy to see makers catch on that high quality audio should be available to, and is being demanded from, the masses, and we are getting much better AIO devices.