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If Apple does embrace ARM, will they do what MS has done, and is trying to do now?
Low end, ARM, but high end still use Intel? If Apple did that, would that confuse and muddy things?
As originally envisioned in Windows 8, the MS approach was one OS, one UI and one API framework which spanned from phones to workstations. This didn't work out very well.
By contrast Apple plans on maintaining a separate OS for mobile and desktop/laptop environments. In the layers above the OS kernel, the upcoming Marzipan product will supposedly be rolled out in several phases, with the final goal of providing a single app development and distribution format that runs on either iOS or macOS. During that that move, the underlying Mac CPU might change.
Obviously there are lots of unanswered questions but this could result in more (not fewer) Mac apps. Right now there's no Netflix app for Mac, and the Mac Twitter app is going away. With Marzipan the iOS versions of those could be ported to macOS. Some early info indicates that would not be a "windowed" iOS app running on a Mac but somehow the app's core functionality would be converted to the mouse/menu interface:
https://insights.dice.com/2019/02/21/apple-wwdc-2019-marzipan-sdk/
Porting apps the opposite direction -- from Mac to iOS -- would also be streamlined. Adobe has demonstrated an early version of Photoshop on iOS. This was probably an immense effort and would essentially be a totally new app which must be maintained in parallel with the desktop version. In theory Marzipan would allow a single app development effort to target both desktop and mobile platforms.
In past Mac CPU transitions, most apps were compiled binaries. However today many apps exist as web apps. Out of the box an ARM Mac could obviously run all those.
For mainstream productivity apps it seems likely that Apple would coordinate with the development community to have those ready by roughly the time an ARM-based Mac appeared.
Re performance or other advantages, in theory ARM could provide a MacBook with the battery life of an iPad.
The desktop space is less clear since ARM has not prioritized that market segment. However from a pure performance standpoint, ARM (inc'l Apple-developed CPUs using the ARM instruction set) has made great progress.
Apple is frequently hamstrung by Intel in both price, availability and features. One example is Intel will not put the Quick Sync video accelerator on Xeon CPUs. Thus the iMac and Mac Pros don't have this and sometimes struggle with H264 video. FCPX on the iMac Pro uses AMD's transcoding logic but it's not as fast. If Apple controlled the CPU they could add whatever architectural features they wanted, on their own schedule.
In theory Apple-developed ARM CPUs would be cheaper, even for higher-end parts, plus consume less power per unit of performance. For workstation-class machines the CPU price can be a significant % of the total parts list. The 10-core Xeon W2155 used in the iMac Pro costs about $1,500. Apple certainly gets a discount but it's still probably a lot of money.
