I have an upper end desktop, a cheap netbook and android phone, a PS3, and a kindle. My family has a shared Ipad and my wife as a reasonably powerful laptop I can borrow sometimes. I use all the others to supplement the desktop, but even together they couldn't replace it.
My desktop is the focus of my digital world. I use it for my programming, for writing papers at school as well as for gaming and media consumption when I am at the house. It does things like run multiple VMs and compile code in a reasonable time that none of the others manage. With its dual monitors and easy ability to handle numerous programs at once, I find it tops the others even for tasks like e-mail and web browsing that the Ipad does handle well. All of those others combined cannot replace what I do with my desktop. Even with her fairly high-end laptop and access to the other tools, my wife often borrows my desktop when she is doing serious writing for her PHD.
The others supplement my desktop for niche tasks. I take my netbook to class to take notes and it is good enough for light programming/paper writing and writing when I am on a trip (and I consider it a small PC, not a PC competitor. Also, I find the netbook quite nice for its good-enough travel computer and in-class-note-taking.). My smartphone is a game changer since it is always with me. The kindle is by far the best device for reading and I now prefer it over physical.
The Ipad though? It mostly has games for the kids since it is simple enough for even my 3 year old to use. I sometimes take it with me to places like the airport or DMV where breaking out my netbook would be a touch awkward and it has a pretty good Go program on it.
In short, I think for people doing serious content creation or serious gaming the PC will live on for the forseeable future.