The smartphone surely belongs on the list of truly game-changing innovations, right there with indoor plumbing, penicillin and the thumbs-up. You can have mine when you pry it out of my cold, dead hand.

But what if we got smartphones wrong? Before the iPhone, cellphones had no single shape or size. You could buy phones shaped like candy bars, phones that flipped open, phones with full physical keyboards. Now every phone looks like the iPhone, which looks like every other phone—a dark rectangular slab.

As...