Apple already has an OS X port for ARM, remember, they based ios off of it. Similarly, Apple already had an OS X port for Intel, when they were on PPC.
For GPUs, Apple can still partner with Nvidia or AMD. Not that many native games are available for OS X, anyway, most just use a wrapper based off of wine. But gaming, was never Apple's target market, in any case. Apple has a lot more power with devs now thanks to the Mac App Store, and Adobe is moving into the cloud anyway with CC, so they can still develop some type of interface for Mac to work with it, from a front-end perspective.
What would be Apple's business case for making an ARM-based Mac? People aren't going to flock into Apple stores wanting to buy one just because the chip architecture is different. If anything, there would be less demand as less apps/games would be supported initially.
The only reason to go ARM would be for cost and battery life. Cost isn't even an issue as an average Mac is $1500+.. the cost of the chip is insignificant. Battery life is already superb. The current Haswell MacBook Air can get 12 hours of battery life, I'd imagine the Broadwell can get 14 hours, and Skylake could probably stretch to 15+ hours. Beyond Skylake, the bottleneck of battery life would be from other components like the screen, SSD, and WiFi.