Think about all of the things that you don't have to carry with you if your smartphone is good enough: camera, wallet, keys, pieces of paper (containing printed maps/directions, emails etc).

It's notable that a combination of Apple Watch + Airpods can fulfill most of these needs, with the exception of being a high quality camera and a few other things that require a larger screen. But that just shows you that if anyone is going to disrupt Apple, it's going to be Apple.