Desktops, whatever platform that might be (and whatever form they take) will always be a platform that will persist purely for the amount of relative computing power they have against other alternative devices. This isn't a dig at Mac's either as at the fundamental level regardless of ideology, external physical appearance and OS it's still on the desktop/laptop computing platform that still has the same kinds of Intel based processors, the same types of memory modules, the same kinds of hardware controllers and usage of all the same kinds of storage devices. But if we're comparing them to Smart devices we're more likely trying to draw a distinction between device form factor rather then the actual platforms themselves. However potentially the issue is not necessarily a question of power if we were to examine most peoples general computing needs unless that is you have the craving for the most current available consumer edge of graphical rendering power whether that be for development or just for video gaming purposes.
 


But it might be worth remembering that we have pocket sized portable gaming devices with vastly more processing power then the PS2, N64 or Wii even when looking at something like the more recent PSP Vita which has a potential maximum speed of 2GHz (2000MHz) via a quad core processor (that’s 4 x 2000MHz) and could quite easily fit in your pocket. Even its maximum potential of 400Mhz on the GPU front runs at something close the kinds of speeds that mid to high end laptop GPU's run at. However keep in mind the standard default settings for extended battery life would look more like 800Mhz for the CPU and 200Mhz for the GPU. 
 



 

When I first started out on a desktop machine I had a full sized monitor and midi tower running a single core 32-bit processor turning 66Mhz on the clock cycles, 4MB of RAM and a 320MB hard drive for Windows. Now we have pocket sized smart devices with quad core processors (as in four processors cores) each running at 1500Mhz (that’s 4 x 1.5Ghz) possibly equipped with up to 16 to 64GB (16000 to 64000MB) of solid state storage. Then consider it will most probably also have 3G/3.5G/(or in some cases even 4G) connectivity, and Wi-Fi. Even more specifically the HTC -X- one has a native resolution of 1280x720.


 

Potentially on the general computer user front for most peoples general computing needs and habits, I don't think its necessarily the issue of processing power that’s the problem. I think what most people need from their desktop or (some kind of equivalent) beyond that is the full sized display and not the pokey miniature screen of a pocket sized smart device and some means of interacting/using it which doesn't require too much effort when comparing it to something like a pocket sized smart device. I guess its just very unfortunate that desktop operating systems have gotten heavier and heavier where even the impressive progression in terms of computing power of the portable variety kind of causes the systems to run very slowly against that relative context of almost exponential progression.

But then its not difficult to see how the tablet is beginning to bring a realistic alternative for delivering that general user experience in a competent portable manner on a larger display via a light and portable all in one device. I guess the next move might be to just have some sort of dock for it that allows you to extend its power and storage capacity whilst at a desk or in the home for a larger screen with the option to use other types of HID's (Human Interface Devices). 
 

 
Potentially it would even still be possible to make the docking system portable too, we only have too look at the laptop form factor and the amount of power that the Alienware's 11 inch netbook has which is fully capable of reasonably high end gaming. With its array of USB ports and a connection for an external display it wouldn't be difficult to use any type of HID a you wanted or any display you wanted.



Its Evolution and progression (...form factor?)

I don't think we should think in terms of a PC (or rather desktop) platform vs. smart device platform as such because if you could cram a desktop and the appropriate amount of processing power into the size of a pocket PDA/phone you would in fact have a smart device. But then technically that in many ways has already happened. 




I didn't actually mention ITX form factor mini desktop systems since the level of progression on that front pales significantly in comparison to devices such as the HTC -X-one smart device. But for anything of the micro board variety that might be worth talking about I'd suggest you look into the Raspberry Pi for true ground breaking high fps GPU rendering power of the desktop variety as a real world example within the domain of public knowledge.





 
Lack of technology maturity and politics usually holds up the process (...but its already here now)


More recently the top end specs of PDA's are finding their way on to slightly more affordable tablet based devices, many will go beyond them. To my mind given that windows 8 has been especially designed to run much more quickly on current and future light tablet devices it makes sense that ITX systems seem a little redundant where there hasn't yet been wide adoption of it. Why introduce a technology into a new potential market for widespread adoption when there is potentially better (and less electrical power consuming) current consumer class technology that has actually matured and has very similar specifications if not better when compared to most current Atom based general ITX systems. Of course this doesn't mean that the ITX standard couldn't be further developed as a more powerful system with better future components, but most current ITX systems are in fact Atom based.

If you think about how much it cost to build an Atom based ITX system (even buying a ready made one) it would cost you a little more if not roughly the same price as a more powerful Windows 7 (in future the lighter and faster windows 8) tablet device with its own screen,  a monitor out and USB ports for using external HID's. There are even more powerful second hand convertible laptop/tablets with their own LCD screens that cost less.