How People Use Computers
Go look at the people who are buying tablets and what they use them for. They buy them to use alongside their computer, not to replace it. The same with Google's soon-to-be-released Chromebook. It's a great idea for some home users, but if you want to play cutting-edge games, or edit videos (hell, even big images), watch DVDs, record videos or music or connect an external device, you're going to want a lot more processing grunt, which means a full OS and a full computer.
If you have to buy one to offer that, most people just won't bother with the cut-down devices as well. As I've said before, low-powered machines aren't worth the small saving (at least, for a main machine). Have you tried writing documents on your smartphone, or a tablet? What use is syncing them everywhere when the best method to write them is still a keyboard and viewing them on a full size screen? Certainly PCs aren't going anywhere for businesses.
Maybe we'll move to a virtualised setup there too, a local cloud, where the computers will be thin clients. This could work for many people, but not all. There's a lot of talk about making data available to someone wherever they are, but in reality very few people in the workforce work outside an office or need access to things outside working hours. There may be endless talk about home working, but it isn't going to change anything any time soon.
The cloud has a lot of potential, but it's not ready yet. You could offload playing games and video editing to remote servers, but you're reliant on two things: service providers and the network. Right now, the network isn't fast enough. Loading a 10GB home movie to edit will take hours, if not days. To then edit and spend another few hours downloading to burn to DVD/whatever (maybe spend days uploading it to another service to share). Not practical. Anyone who has tried to do anything complicated with online photo editing will know they're not worth it either.
Just uploading the 100+ photos you take on a day out from a camera in the current megapixel race takes hours now. It's too slow to support moving large amounts of data. Give it five years and we might see some improvement, but even then only to a percentage of the population. In the meantime, big files stay local.
Let's assume, for a second, all your applications eventually get hosted in the cloud. You can play games, edit videos, edit photos and do all the other things people do with PCs, all via service providers. I buy a bit of software, I can use it as much as I want, whenever I want. Will these service providers do the same? Will I have to pay monthly regardless of how much I use it? Will it be a pay-per-use scheme, how often will I use it when I'm constantly thinking about the cost when I fire it up? Maybe you'll get unlimited access to a range of apps for x a month, but what about if you're provider doesn't have an app you want? Does that mean signing up with another one? Costs suddenly become much harder to manage.
Something that no one seems to be thinking about is the long-term flexibility of their files and data. What happens when you want to move suppliers? Even if you can get all your data out (which very few seem to be offering), how do you transfer it, en-masse, to another provider? The answer is likely to be, with a lot of pain, if at all.
Do you think Apple, Google, Microsoft and the others know this? Oh yes. Once you're in, you're pretty much in for life.
And that's before we get to security. There's been a lot of high-profile attacks lately and while I'd like to think most service providers are better than to get hacked using a fairly basic SQL injection hack like Sony were, I've actually read a couple of first-hand accounts by hackers and those who caught them. The thing you learn is that, to be a great hacker, you don't need to be a great coder, but what they all seem to share is perseverance and lots and lots of patience. You know when you think 'no ones going to bother doing this?' Well these are the people who will, and do. A lot of hacks come down to trying things over and over and over.
The other thing you learn is that it doesn't matter how good your security, they will find a way in eventually, they only need to find the smallest chink in the armour to break in. I'm guessing the bigger the prize the more likely they'll spend the time, so going after a single person's files (unless they're strategically important) is less appealing than, say, a huge multi-national. Which means your files hosted by Apple, Google, Microsoft and others are much more at risk than your data protected using your own crappy security measures.
So the online panacea that we're all being promised comes with far too many pitfalls and, while taking it all into the cloud may mean we can live a far more joined-up life, most people don't need it, don't want it and will find it doesn't offer them the flexibility they want. So the PC is going to be with us for a long time to come yet and it'll remain top dog.