To make such bold statement either u r a lousy photographer with dslr or u never use a dslr before. At the end of the day its the man behind the lens that matters.

To take those wonderful shots with iphone on billboards are famous photographer who know exactly what is the strength of the camera and take the shot. They didnt use it to take pictures that show weakness of a camera phone like potraits, extra wide landscape, moving subjects or dark environment. These photographers can squeeze out 100% potential out of the device they have. Try search youtube for "pro photog with cheap camera" by Digitalrev.

There are many dslr users who are just lousy. Shooting in auto mode might as well use compact cam or phone. But you can't generalize all dslr users like that.

It all depends on what you want. U want portability and simple to use then camera phone. U want high quality picture in high res then dslr. If u just want to  selfie, instagram or fb the photo, iphone is good enough to view on portable device. If u want to pixel peep or bokeh then dslr. There is market for both segment. Iphone can never replace dslr but replace compact camera is a true fact. Mirrorless taking over dslr market share is also very true as well but will not replace totally.

At the end of the day, the best camera is the camera that is with u when u wAnt to take that shot