I am quite sure that macOS runs quite fine on an A12. Like back in the PowerPC times, where Apple would build OS X always on x86 chips too, they certainly keep testing macOS on ARM hardware. Even if they don't have any imminent plans to release an ARM based Mac, it is a great way of testing your software for portability.
Of course, but without any software. Microsoft has done some interesting stuff in that regard so maybe there is an easier transition than getting all third party developers on board, but it's a tough nut to crack.
Apple have experience of that, and I mean a lot. The first C compiler I used was the cheap student version of Metrowerks that only compiled to 68K, I was on a PowerPC machine at the time. Mac OS X had a Mac OS classic compatibility mode until around the time the Intel processors replaced PowerPC ones, and early Intel machines could run apps compiled for PPC.
Apple is going to have a good ecosystem of ARM software very soon. The whole reason they pushed their Marzipan initiative with the ability to seamlessly port iOS (ARM) apps to macOS (x86) is so that they can also do (iOS >>> macOS) on ARM.
By asking developers to port from iOS apps, they have the side effect of getting developers to implement "fully featured" apps, just with a mobile UI. Adobe has already promised they'll be doing this with photoshop and Illustrator, releasing in 2019.
Others will follow suit. Not only will this strengthen the feature set of iOS apps, but it will strengthen the iPad Pro app ecosystem, and apple will simply link the iOS and Mac App Stores, allowing for "universal" apps.
Marzipan will translate them to x86/64 and later when Apple switches to ARM, Marzipan will simply keep these apps on ARM, and change their UI to match.
If you've been watching Apple closely over the last few years, you'll notice that they're consolidating their API's across macS and iOS. Things like Metal and AppKit and UIKit and so on are beginning to converge. This is all setting the stage for Marzipan.
They announced the "first half" of what they intend during the WWDC 2018 keynote. But the second half will be announced only after they reveal ARM Macs, and then "surprise - all the iOS apps you see now are available here, too. They automagically get UI adaptations for mac!"
Yes, the challenge is the software. Not so much the native Mac software created with XCode. That would probably rebuild at a button press. The elephant in the room are VMs. Many Mac users (for example me) are running x86 based operation systems inside VMs on their Mac. This wouldn't work without a dynamic translation like Rosetta. But as Apple is making their own processors, I would consider it a possiblity that they add some hardware accelerators for a Rosetta style software to their chips.
While I understand running VMs are important for you and many developers, people like us who want to do this have to be a miniscule share of Apple's market. I wouldn't bet on them spending much time on it.
You are right that developers are only a small part by volume. However they are the most important users in an environment, as they the ones creating new applications. Same with other power users, who require a VM. Driving them away from the platform could have a much larger net effect. That Macs got much more popular after the switch to x86 hints at that the compatibility isn't quite unimportant.
So I do think that Apple is probably going to ARM, but they need to be very careful in the steps they are taking and they need to have a really compelling offer so that users are willing to go through the transition.
