I think that the point being expressed is that ARM will be "good enough" for many traditional desktop applications; it doesn't imply that ARM will supplant Intel on the desktop. My opinion is that Apple will at some point build an ARM powered desktop slotted in price below the Mac Mini, as well as a Mac Book Air class notebook powered by ARM that might be an iPad and separable keypad.
Regrettably for us all, Apple (desktop) is joined at the hip to a certain extent with Microsoft as so many users still require Windows only applications on Macs.
Probably no. If Apple to built ARM based Mac, that means we get intel based Mac and ARM based Mac at same time, that also means all the application need to be written with ARM code and x86 code. Which will confuse customers like Microsoft did with their Windows 8 Pro and Windows RT.
The reason it won't happen anytime soon is that ARM is not powerful enough to emulating all x86 applications. ARM based processors will also have hard time to match the x86 version of application in terms of performance. That could be deal breaker for most people.