Why won't GNOME be a player on tablets or phones? It is a pig. Gnome Shell might be designed to look like a tablet or phone but the GNOME stack is a bloated hog. On a tablet 1GB of ram is the max currently seen in the wild and that only on the premium products that won't be running GNOME. No, they would have to break in at the bottom and look at the specs on those, CPU speeds at 1Ghz or lower and 256-512M ram No way GNOME is going to run well in that.
It gets worse, it is probably true that to survive the checkbox problem there would need to be smartphones too, their hardware and available battery is even more restricted.
Haven't really looked at KDE lately but I'd bet their footprint is just as horrible. And so will Windows 8's footprint be too heavy to succeed in a resource and power constrained environment. So it is likely to remain an iOS vs Android world for a few years off the desktop/laptop.
We had a choice a decade ago to keep our stack lean and mean and able to run on older hardware. All the bright kids with big ass developers desktops said screw that, lets bang it out in Python or layers upon layers of C++ frameworks and make it pretty, 90% complete and move on to something more interesting because it runs plenty nice on my machine and it will run OK on most people's machine in a year when the distros catch up. But had we kept the ability to run on older hardware, now when new tablets have the power of desktops from a few years ago we would have been in a position to rule the world. Alas.