I was just watching a snippet of the 2005 WWDC keynote where Steve Jobs announced the Intel transition. There are important things he announced there that really taps into why Apple is doing this or is planning to do this.
Up until the Intel transition, Apple went through two major transitions:
68K to PowerPC
Classic Mac OS to OS X
By 2005, Apple was in its 3rd year of OS X releases. Steve Jobs pointed out at the time, OS X would be the flagship platform for the next 20 years. Taking into account, this would have started with the initial release of OS X Puma 10.0 in March of 2001, (gonna start with the public beta), we are now in the 18 th year.
That means by 2020, the desktop would be ripe for the next platform transition. This seems to be written in the books at Apple for a long time to always have a platform transition plan some point.
Another thing to look at is each processor transition has been at least every 10 years. But, it wasn't just about age and change for change sake. Steve said it was about performance per watt and the ability to design future Macs within a certain thermal envelope - 2008's MacBook Air is an example of this.
Looking at it with todays products, Intel is good, but in the eyes of Apple, not good enough for what they really want to do in producing products (guess), much thinner than even todays Macs, even faster, power efficient and better battery life.
So, the view is, Apple is setting us up for not one, but two major product transitions in 2020. Just like PowerPC, it will be multi year. Although there are fears this one is gonna be like PowerPC, I think Intel might enjoy a longer shelf life beside Apples potential A-Series products. Who knows, it might be even be so transparent where, they sit beside each other for a few years until you start seeing standalone A-Series MacBooks, Mac Mini's, entry level MacBook Pro's.
Similar to the double life OS X led on Intel, I suspect Apple did the same with macOS on A series. The first time they probably got macOS running on A Series was with the release of the A7 in 2013. I remember Phil Schiller describing it as desktop class performance. Apple's A11 Bionic development goes back to 2014. This means, they are already 5 years ahead in terms of the road map for the A series. Imagine what they have ready for 2020?
By WWDC 2020 we are gonna see a fully developed, optimized brand new Apple OS, not only for the desktop, but mobile products too.
The pieces themselves kinda give a clue as to what Apple wants to make: APFS, Apple Pencil, Touch Bar, Touch ID, Taptic Engine, T1, AR, A11 Bionic.
Don't be surprised when major third party apps are shown off fully running on this thing. The NDA's are already signed and think clues have been leaked, thats why Gurman knows about it.