Once we get wireless displays in our phones (and cheaper storage, which is coming), that will mark the final straw, everything will be phone based.
Right now we try to bring the data to the phone, instead, reverse it, put everything on the phone and run everything from that. With this method, you always have your data, instead of always retrieving it from elsewhere. As the phone becomes more and more the primary device, this makes more and more sense. If you haven't seen it, take a look at Clambook. Now adapt that same system (wirelessly) to a desktop and tablet. They won't even be a thin client, they will simply be extensions of your phone.
This means less services to pay for since you only need internet on the phone, as well as fewer items to maintain. It ends up far cheaper and more efficient as you don't even need to buy a tower anymore. As it stands, the only thing really holding it back is a UI that works on a desktop, any guess as to where Microsoft and Ubuntu think we are heading?
Desktops will remain, but they will become more of a tool (like they were 30 years ago). Gamers, artists, etc, anything needing more power will stay on the desktop for at least a bit longer. Some things need the extra power, but the average store bought PC that is mostly for browsing the internet will be dead. The same applies to game consoles, they will go back to being just a game system for more hardcore gamer as most will play from their phone connected to whatever peripheral they choose.