As someone else here told me (maybe in this very thread), MS will be spinning Win10 onto ARM as a part of their big Internet of Things push. It won't be a full, standalone version of Windows 10 (at least not starting out), but Windows on ARM will be around in some shape, form, or fashion.
ARM is very quickly becoming a competitor for Intel and x86. Right now, it doesn't work as a direct replacement, but who knows what the future'll bring. While MS isn't supporting it directly at the moment, they're far from ditching it entirely.
But the Internet of things is not related to tablets or laptops as I understand it. I've not heard that MS is including the ARM code in a unified code base, but that doesn't mean they didn't say or that it won't happen. I still think though an ARM based tablet or computer running windows is dead. To bring this back onto the topic at hand, I've used this reasoning to support my supposition that for many (most?) consumers this will be a major issue that will drive down sales of Macs.
I see no reason why apple would chose to shift to a whole new platform, it makes no economic, business or common sense. When Apple went to the PPC, they did so because they needed to shift to a RISC platform using Moto and IBM, They believed at the time, this will propel them in front of Intel in performance and power, which it did I believe for a while.
The second transition was needed for their very survival. IBM over promised on the G5, and under delivered. They had trouble providing apple with the quantities they needed to fulfill computer orders, they were unable to provide a low power variant for laptops (even though they promised it they would), they failed to deliver a 3GHz version (back during when cycle rates were king).
Since the G5 was an IBM only creation, Moto wasn't going to or able to help and Moto was getting out of the PPC chip business, i.e., spinning off the division as freescale.
My point is the prior platform changes were born out of Apple's need to keep up with the competition or more so to survive.
With a move to ARM processors, none of that is needed, Apple has enjoyed great Mac sales while the industry languished I believe in part because of the Intel chipset. Move off that, and you remove a major advantage.