Most of Apple's offerings are practically commodities, like the glassware you drink from. You don't "upgrade" your coffee mug whenever Starbucks introduces a new reusable mug that holds more liquid than their last iteration. You keep using what you already own unless it leaks or breaks, because, you aren't lacking technology that gets the job done. The coffee mug is considered a solved problem in this day and age.
Likewise, the computer (desktops and laptops) market is pretty much saturated. You don't really *need* a faster computer to send emails or make a voice call. The majority of consumers are happy with their PC/Macs today, unless it no longer functions as intended. Over time, people should see less and less incentives to switch to, or upgrade their Macs to, the latest best Apple devices unless they actually need to replace what they already own, or they are going through changes in their day-to-day work.
As it gets harder to justify new purchases on technical specs alone, what do you suppose will drive people to buy from Apple (either as switchers or repeat customers) besides design, that will keep the company growing sustainably?
Apple is an affordable luxury brand. With a proven track record in high-fashion, I believe Angela Ahrendts is a great fit to contribute to the design of the complete Apple customer experience, which requires a great level of attention to detail in major points of interaction (billboard/online ads, store location, store layout, sales rep etiquette, packaging, Genius Bar, etc.)