I don't foresee the changing from x86 to ARM as an issue as nowadays more high level languages are being used and even low level ones that utilize specific platform features are now moving on to a more abstracted methodology where platform independence happens at a much lower level.
ARM is fantastic, let alone that it is RISC, it's very power efficient and most of infrastructure networking hardware does utilize ARM. As for the PC/Desktop scene, the fine hair line that distinguished mobile implementations of a PC and a desktop has disappeared. Sure thing, VLSI boards beat laptop form factor in expansion and capacity, but that argument can now be kept in the servers' room.
Mobile computers are more powerful than ever, cloud computing is emerging in scary figures, if you have a need for super computing experience you can now have a NVIDIA card that has close to 1024 cores of parallel computing at your disposal (yes that's inside a laptop). Computing is not centered around software needs anymore, it's all about usability.
And to be honest, I don't see mobile computers with at least 15" monitor and a full QWERTY keyboard disappear any time soon within the next 7-10 years, simply because the whole computer user experience is largely still centered around the keyboard, and that's not necessarily a bad thing, but I'm confident that unless one figures out a method of input that at least provides the control, the granularity, and the accuracy of a keyboard, the traditional form and shape of a computer won't simply just go away because of a change to the processor market. You'd certainly witness very powerful and very mobile computers, but they will still be the same in form and shape. Tablets and what not are very much needed tools, but they are never meant to replace your laptop or desktop, they just facilitate another type of usage for a computer that fits particular real life situations.
In a nutshell, if you are dreaming of the one computer solving all of the problems then you should be looking at nano technology in depth, and other forms and methods of computer input. If a computer can't be under your skin and you can't read off of eye lenses, which is going under research only battery is the problem right now, you'll always have a need for several forms and shapes of a computer that address different usability situations.