Apple has been on this path for quite a while now.. look at what they've done in the past few years.
They've sealed a lot of the lineup now.. the Retina Macbook is sealed, Air is of course sealed, the 21" iMac is sealed, and I'm sure they'll eventually seal or get rid of the non Retina MBPs, and the Mini will be sealed soon, leaving just the amazingly outdated Mac Pro to rot away.
At the same time they've done this they've also emphasized form over function, they made the iMac a bit thinner and lost the 7200RPM drive, optical drive, and even beyond that they've never adopted Blu-Ray, took ages to get USB 3.0, and generally tend to put little thought into the graphics of their computers. I mean a 512MB GT 640M is comical for a $1300 computer.
Power users are dropping Apple like a rotten piece of fruit while the iPhone/iPad users are now getting Macs and they want them to be more and more like big iPads with a mouse and keyboard.
ARM simply isn't powerful enough to do a whole lot, luckily they've largely pissed off most of the power users and they've either done the smart thing and jumped ship or they will when this happens.
There is no way they'll be able to emulate Intel processes with an ARM chip that's gonna be a fraction of the power... so they'll probably have to come out with a new OS that's dropping all prior support.
It will probably launch as OS 11 or even just a brand new OS all together, dropping the Mac or something and Mac OS dies off, could even see them renaming their lineup. OS 11 will probably be dumbed down to unimaginable levels, I see full screen apps only, app store apps only, and a very sealed OS with maybe no file manager.
The writing on the wall is here, if you use your computer for more than Facebook and Angry Birds you might want to look to a new company.