"Good design" is in the eye of the beholder. For example, personally, I find Apple's designs to be a bit contrived, pretentious, and having the feel of trying too hard to be simple. However, a lot of people don't agree. In particular, people who: a) have a lot of disposable income or b) who want to attach themselves to a "sexy" technology brand probably either don't even think about Apple's designs, or else find them attractive or c) both of those.

In other words, they became the most valuable company in the world, even though they have a tiny share of the market in almost every segment they're in other than the mobile handset space, because enough people are willing to pay the "Apple Tax".

And now they're dropping skew-morphism, which demonstrates the point.

Apple grew because they made products people wanted at opportune times, executing brilliantly. Indeed, though, Apple proves the point I was making: They executed well and made products people wanted, so every decision they made goes backwards into justifying itself as good design. Speaking of Dustin Curtis, that the iPhone was 3.5" was held as some holy number of smartphone size (rather than being the size that Apple cemented themselves into back when building a 3.5" LCD was actually fairly expensive). And then 4.0" became the ultimate in design. And, soon enough, larger sizes will become the pinnacle of design.

Were any other company responsible for making the new Mac Pro, it would have been met with devastating reviews. Correlation, however, and people somehow reach to find a way to make a case for why it is actually a good design.

Good design isn't the only thing that's made them so valuable. They've also made a lot of very smart strategic decisions to do with their supply chain and finances, which enables them to execute designs that their competitors can't afford to.