As long as you have the ability to install other DEs, like Xfce, "Ubuntu for desktop" will never disappear. That's the difference between Linux and locked down operating systems like Mac OS X.
I've never liked Unity, but that's totally irrelevant to me. I can just keep using Xfce (or any of a dozen other WMs/DEs) with Ubuntu.
PCs are disappearing because most people who use them don't actually need them. They don't need a truly general purpose personally controlled computer.
Linux users know they need a personal computer, for software development or some other high-value purpose. These will be the last PCs to be replaced by tablets and smartphones.
Unless of course your position is that Ubunutu is effete and it will be the Mint and Gentoo where the Last of the PCs makes it's stand.
As for gaming they are now starting to release linux versions, well at least Steam and Blizzard are.