Important bit to note here: the two year timeline is probably only feasible for low end devices, like the MacBook and MBA.
Towards the higher end, ARM can't hope to field anything in that timeline to compete with even todays i5 or i7s (or corresponding Xeons). Some people do use this kind of CPU power.
I don't have a guess at what Apple is actually going to do, but the Retina rollout is a plausible model. Even 5+ years after the first Retina product, it's still not available across the lineup.
I don't think the switch would present a significant problem in marketing or for developers, so it would purely be a question of having the chips that fit the products. The Macbook, as you point out, is basically already there.
Towards the higher end, ARM can't hope to field anything in that timeline to compete with even todays i5 or i7s (or corresponding Xeons). Some people do use this kind of CPU power.