The easiest way to build a big business is to solve the hardest and most important technical problems. The market position would be defensible due to hardness, and the market size would be big enough due to importance.

There's many other approaches to build a big business though. While Apple's workers and investors believed in Apple's approach, Nasty Gal's workers and investors believed in Nasty Gal's approach. Capitalism is the judge and redistributes capital to the approach that works.

Apple's workers and investors now have more capital, so they can start more hardware tech companies, while Nasty Gal's workers and investors have less capital, so they can start fewer online clothing store companies. It's inefficient, but it works out in the end.