There's been various chatter about this in the forums here at MacRumors.
Here's some of my issues with going to ARM.
1. Lack of windows compatibility. I need and use windows for my job and I'll not have the virtualization ability, never mind Bootcamp. Plus, Apple enjoyed a large spike in people buying Macs when they transitioned over to Intel. They risk alienating a large core of customers if they leave the X86 platform.
2. Performance, will the ARM perform just as well as intel running the multithreaded applications? True the iPad Pro's performance is exceptional, but its running iOS with one, maybe two apps (with split screen). How will it perform with a desktop OS and file system, with many apps running at once?
3. Developers, Apple will be asking a lot of developers to rework their apps and will we only be able to load apps from the app store at that point? Will the OS be locked down like iOS and we'll lose the ability to load Carbon Copy Cloner, or other applications that forego the MAS and apple's 30 percent slice of sales? Will large developers get on board, i.e., Adobe, Microsoft, etc?
4. Will Apple emulate the legacy code, and how will that perform, i.e., when they transitioned from PPC to Intel we still had the ability to run PPC apps, how will the transition work, can the ARM processor handle the emulation. If there's no emulation, that will make it almost impossible for apple to sell an ARM based Mac.
Personally, I find it difficult to justify spending 2k on a 15" MBP that's running a broadwell chip in 2016. I'm definitely sure, I'll not spend 2,000 dollars on an ARM based laptop. While my single sale may not mean much to apple, I'll be one of the people who chooses to walk away from the Apple computers if they move away from the intel chipset. The competition has already leaped frogged Apple in design, performance and price. Apple needs to not only catch up but surpass them, but I think embracing the ARM platform is the opposite - just my $.02