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.
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!"
But it's not the point, main point is that I think statement that current A12 is close (to whatever reasonable %) to top desktop chips is simply not true. I didn't see any concrete evidence of it, and everyone just seem to like this idea, that's why it was talked much lately. I'm all for great advantages and all that, but I still must use common sense, 3-4W vs 150+ W - that's too much.
Per-core power consumption on that Intel chip is 165W/28cores = 5.89W
Apple is pushing similar IPC at 3W instead of 5.89W. In that context, it's not so unbelievable, and it's still really fucking impressive.
And that's my point, Apple already has very impressive thing going on, no need to smudge it with sensationalists statements.