If the Mac market is going to shrink to the size of the current market for Mac Pros, perhaps that will be the only model they keep alive?
I don't agree with these tidings of doom for the Mac. The fact is, a desktop computer is still the best job for many tasks, and a touchscreen tablet wont replace them unless it becomes a desktop computer. The iPad form factor/input mechanism is inherently flawed for certain tasks, where the desktop excels at them.
I believe that with computing devices (PCs, tablets, smartphones etc), the price you pay is linked to the amount of time you will spend with it. Smartphones are generally the cheapest and you spend relatively little time with them. Usually a few minutes, maybe 10-15 minutes tops. There are two reasons for this. The first is that by definition, it is a travel device, and while travelling you generally don't need to sit down for long periods and do something, you need some information quick. The second is that it is too small to do any real work or entertainment on.
Tablets, assuming they all follow the basic form and input of the iPad, are in the middle price wise and also time wise. You will often spend 10-15 minutes on it in one go and anywhere up to an hour or two if you are watching a movie, reading a book or creating something. However, it is very rare that you will use it for over two hours at a time.
PCs are at the high price range (or they will be when tablets take off). You often spend anywhere from an hour or two up to an entire day in front of one. You usually spend a lot of time working on them or playing on them.
So that is where I see the three devices. I've already explained why the smartphone isn't used much, but for why the tablet and PC are where they are takes a bit longer. Ultimately though it is down to a few things: ergonomics, power, accuracy.
If you are working anywhere for a long period of time you need an ergonomic workspace. I know this as well as anyone after suffering a bad case of RSI when I was 17/18. This was largely down to me working on laptops, where the screen was low, the keyboard was a bit cramped and the edge of the laptop dug into my wrists. Since then I have moved back to using a desktop as my primary machine, switched to an ergonomic keyboard and got a decent mouse.
Tablets are far from ergonomic. You can get into a position where you are comfy consuming (reading a book, surfing the web, watching a movie) quite easily, but for creation is is a bit harder. They are worse than laptops in that the input and the screen are even closer together. You have a choice between your arms being in an uncomfortable position and your neck being in an uncomfortable position.
There isn't really a good solution. You could wall mount it at eye height and use a bluetooth keyboard, but then your arms have to reach up to touch. You could use a keyboard dock but your head has to tilt down. There is no good way to create for long periods of time on the iPad. However, for creative tasks in short bursts it can work very well (eg writing a song where you every so often use the iPad to jot down the notes and/or lyrics).
The ergonomics of the PC have been well honed over many years and there is lots of advice and lots of products out there to help you. It became important because of how long people spend in front of a computer.
Basic law of technology: new technology costs a lot to make and isn't very efficient. This is why all new technology goes into the high end products before trickling down to the lower end. Faster processors go in large towers before they get cheap enough and efficient enough to go into laptops. This is why the current Mac Pro allows up to two quad-core 2.93GHz processors, the current iMac up to one quad-core 2.8GHz processor, the current MacBook Pro up to one dual-core 2.6GHz processor, the iPad a single-core 1GHz processor and the iPhone 3GS a single-core 600MHz processor.
This law won't change, so the bigger, more expensive devices will always getter the new stuff first. It will probably be 5-10 years before the iPhone has the processing power equivalent to the current top of the line computer, but at that point the top of the line will have 5-10 years more advanced technology.
There is also the power in the form factor. A tablet is, for all intents and purposes, a hand held device. Therefore it needs to be small and light. You're not going to see a tablet get much bigger than 10-12", before it gets too big and too heavy to bother with (much like a 20" laptop). For many things a bigger screen is essential. For any sort of pro media editing, the bigger the screen the better. For programming, the bigger the screen the better. And not only that, more screens can be better too. And this isn't just a geek thing, many people who have these setups aren't tech savvy, they just need them. You are never going to have a 27" iPad with support for 2+ displays. At that point it isn't a tablet and more of a paving slab.
Ultimately, a PC will always have faster processors, more storage, more RAM and more form factor freedom than a tablet, in the same way tablets will almost always be the same to smartphones.
Accuracy is important for many lines of work. Not only how accurate you can be but how often you can be that accurate. You are always going to be more accurate on a hardware keyboard due to the tactile feedback (at least until we get tactile feedback on multitouch screens). The mouse is always going to be more accurate because of the disconnect from the screen meaning you can see exactly where it will act.
For tasks that require accuracy, this is a killer. The only way for a tablet to get the level of accuracy would be to either use a stylus or a mouse, neither of which are ideal and would hamper the tablet.
At the D8 conference Steve Jobs said the following:
When we were an agrarian nation, all cars were trucks. But as people moved more towards urban centres, people started to get into cars. I think PCs are going to be like trucks. Less people will need them. And this is going to make some people uneasy.
This really is the perfect analogy. Cars are ideal for almost all everyday tasks. You can go to work, go shopping, take the kids to school etc. Trucks, vans, lorries etc. are still needed though. They are used by workmen to hold their tools, by companies to transport goods and by everyday people for handling tasks like moving furniture.
Trucks are heavy duty machines, ideal for the tasks they are used for. Cars are for your everyday tasks. PCs and tablets will be the same. Despite all the things mentioned above, tablets are still better for 70% of computing tasks. They will become the more dominant form of computing, which most people use for browsing the web, talking to friends, sending email, playing a game etc. Of course this will depend on various things. With the iPad it needs both the multitasking in iPhone OS 4.0, and a mail client that isn't pretty much useless.
PCs will stick around. Fewer people will need them. I expect tablets will wipe out the sub-$900 PC market in the long run. PCs will become higher end items, for those who need them for extensive work or for specialised tasks. I doubt Apple will be hit too much by this. The Mac Mini will probably die, as will the lower end laptops. Companies like HP and Dell may temporarily shrink, depending on whether their tablets take up the slack of the fall in low end PC sales. Ultimately though their profits should increase as they stop selling as many products with razor thin margins. I think if HP pulls off the Slate with WebOS it could quite easily retain its role as the dominant player in the computer industry.
I have no doubt that tablets are the future, I'm just incredibly sceptical that PCs are the past.