Disclaimer: Currently I am employed by Microsoft, but my views and thoughts are still my own. The reason I joined Microsoft was, the work Microsoft have been doing for last couple of years in Open Source Space. Today I am a advocate for Open Source representing Microsoft.
Not the first time
The shift from x86 is not something new. The initial version of Linux was written of i386, but today most used platform where Linux is used might just be ARM.
ARM has been part of Smartphone since almost the dawn of modern Smartphone. We did have soe MIPS based Smartphones, but at the the those devices were called Handheld PC. But both the iPhone (1st generation) and HTC Dream (first Android) had a ARMv6 CPU. Latest iPhone and Android devices are running ARMv8
Even my first Smartphone (or rather a Handheld PC) was a HP iPaq 614c with a Intel XScale PXA270 ARMv5 CPU, running Windows Mobile 6.1.
Last October, Microsoft released a new ARM based Surface tablet, Surface Pro X. Pro X is Microsoft second attempt at main stream ARM.
First attempt was when Microsoft launched the Surface Tablet in 2012, it had a Nvidia Tegra 3 ARMv7 CPU.
The Tegra 3 based Surface a had cut down version of Windows (called Windows RT). Windows RT was designed to run on ARM based devices, and could not run traditional Win32 based applications.
The problem of Legacy
Windows has always been associated with running Win16/32 applications since start. Win32 is designed to run on x86 architecture. Back when Windows RT was launched, people wanted to run these traditional application, that I will say is why Windows RT really failed.
I am sure, as me everyone who ever used a Windows RT device felt a bit constrained. And the reason you don't feel same constrains with a Android Tablet or iPAD, you don't have exactions. When iPAD was launched it was launched as a larger iPhone. While Windows Tablets were launched as scaled down version of full laptop. My guess is Microsoft had decided to instead put Windows Phone on it instead, and marketed it as a larger version of phone it might have had a better success.
Windows still has largest share of OS market, and that is what is going to be the biggest challenge for running Windows on ARM. And that is why Microsoft has created a Win32 emulator for new Windows 10X (shipped with Surface Pro X).
The largest usage of Window is in Enterprise, where there are abundance of legacy application, some of them still running on Win16. Microsoft has to cater to these needs, and that is why we have some attempts from Microsoft which seems just half way.
Learn from others
Apple is on other hand is known for shift directions, and doesn't mind breaking backwards compatibility in the path to innovation. Also Apple doesn't have the same compatibility challenge as Microsoft is facing with Windows.
Just like on Windows, Apple started to enable developers to port their iOS (more specific iPAD) apps to macOS a couple of years back. But with Project Catalyst you will be able to run majority of iOS apps on macOS.
It does help if you already have a well established app echo system in place. Back Microsoft launched Windows RT, Microsoft did not have a established app store (well Microsoft still does not have a established app store). Apple has been forcing users to use app store in macOS for most of the major updates and other essential apps like Xcode.
On other hand, due to the legacy it has been hard for Microsoft to do anything similar. Users still expects the same workflow they have been using since 90s.
Switch from PowerPC to x86 enabled the popularity of Hackintosh. With the move to ARM and Apple Silicon, some think Hackintosh will die when Apple ends the support for x86. But I believe, with the rise of SoCs like Raspberry Pi running ARM, we might see the shift of Hackintosh from x86 to ARM.
I do think that Apple has a good chance to succeed with macOS running ARM. As Apple is already known to throw out legacy out the window, when ever they have a new idea, users are not going to have to much objections.
Apple choose a good time to experiment with ARM in desktop and laptops. With the results from Microsoft's experiments over 8 years. With the current advancements, ARM is now capable of delivering laptop level of performance. My guess is an Cortex-A76 CPU probably has similar performance to a modern Intel Pentium or maybe even more. Plus web and mobile apps does help.
Sky is the limit
There is a large possibility for Apple to succeed with switching to ARM. But there is also a possibility of failure. When Apple switched from PowerPC to x86, they were switching to something that had a proven track record for desktops and laptops. While ARM still need to prove if it can replace the domination of x86.