As a dyed in the wool Apple fanboy, I will be watching this with great interest.
I think it's important to note that-while Mac sales are small compared to iDevices-they still serve a critical role both inside and outside Apple as development platforms for iOS.
If this is true, it will make app development even easier since they can run natively and not in an ARM emulator.
With that said, I haven't liked the overall direction of Macs over the past few years. The focus seems to be on smaller, thinner and lighter at the expense of useful things like ports. I don't want to carry a dongle just to be able to plug in a flash drive, and I think Magsafe is the greatest thing since sliced bread. So, for now, I'm gladly still sitting on a mid-2012 15" non-retina MBP. It has a full complement of ports, including ethernet and Firewire. It has 16gb of RAM-which I upgraded myself from the 4gb it came with-and a Samsung EVO SSD(also self installed). I have the matte "anti glare" screen, which I much prefer to the glass screen and is no longer available on any computer. It serves my needs well, and I expect it to continue doing so.
Also, the Mac Pro is grossly outdated as well as being outdated and overpriced for what it is. Plus, the computer has only minimal internal expandability. I have upgraded my 2010 Mac Pro(5,1) to where it beats the 2013 Mac Pro(6,1) in every measurable processor metric. I fitted dual hex core 3.46ghz(turbo boost to 4ghz) Intel Xeons, which beat the single 12-core offered on the top end model in clock speed and in raw processing power as measured by Geekbench. I COULD put a much better GPU than is available in the 6,1 in my Mac Pro(I haven't done so to retain legacy compatibility). The computer boots and runs off a PCIe SSD that came from a Mac Pro 6,1. I have USB 3.0 via a $20 PCIe card. I have 32gb of RAM, which is plenty for me, and it's a lot cheaper to put a whole lot more in it since I have 8 slots rather than the 4 in the 6,1.
All of that aside, most 80s computer companies that underwent a processor transition went under. Apple has switched architectures twice now, and in both cases the transition was as seamless as possible. Apple maintained 68K emulation up to the last version of the "classic" OS(and it was invisible unless you looked for it), although to be fair OS 9 had become a bloated disaster and some core parts of the OS were still written for 68K and run through the emulator. PowerPC emulation was also essentially invisible in OS X 10.4-10.6 but worked great just as long as you weren't running software that required G5-specific instructions. If any computer company can pull off a full blow transition, Apple can.
With that said, ARM WOULD present its own set of challenges. Windows would run fine as it is now ARM native. x86 programs could be interesting, though, as Intel has traditionally been reluctant to allow emulation. There's also the fact that there are a lot of 64 bit programs now, and AMD would have to agree to allowing -64 emulation.
I'd also have some concerns about macOS becoming the "walled garden" that iOS has been from day 1. Apple has locked down OS X/macOS more in the past few years, but its easy enough to bypass if you know what you're doing(you don't even have to do Unix magic in terminal-they just hide some of the security settings pretty well, and since they're overall good I tend to only lessen them long enough to do what I'm trying to do).
Everything is speculation at this point, though. It might not happen, but then the rumors are strong enough that it PROBABLY is.