From my point of view, the principled shader is meant to be integrated into a workflow pipeline, or to make more physically-accurate materials. This is good for most of the things that cgcookie users need it for, but what if you’re just making something quick and simple? If I need a piece of paper in the background of my render, it would be much easier to just use a diffuse shader than confuse anyone who I send my source files to with an entire ubershader. Also, if I’m looking for something more artistically styled than realistic I might still use the old shaders. For most cases yeah, the principled shader is probably going to take over. But cycles classic might still have reign in some respects.
Honestly though I haven’t tried out the shader yet. So I’ll just have to wait and see.
