actually you can compare the performance in real world scenarios reasonably well using an os that runs on both argitectures like linux for example. Sure compiler performance will make it differ a bit but its a fair enough estimate. I don't think comparing an iphone like that will work as the you can't just throw a linux distro on there but you can compare a raspberry pi with a desktop processor reasonably well if you give it the same workload on the same os.
That being said while it would not surprise me if arm will take over the desktop market at some point the current architectures are really not there yet. ARM is however creeping into chromebooks and chances are that it will slowly creep up in the server and laptop market eventually getting to be the main architecture. Not anytime soon but it may happen.
Remember arm is slowly creeping up into chromebooks and servers so that may eventually reach the point where they will hit the desktop space. Of course not anytime soon and the current designs are just not made to run at the power budget of a desktop processor but given enough time it may eventually get there.
They have their uses, but there is no reason to use ARM when power and cost are a non-factor. They are showing up in Chromebooks because those are meant to be cheapo laptops with limited functionality.