The iPhone is arguably the most valuable product in the world, representing the backbone of Apple Inc.’s half-trillion-dollar hardware business and undergirding its software-peddling App store. It remains the envy of consumer-product companies world-wide.

If history is any indication, though, America’s favorite handheld device will someday take up residence with the digital camera, the calculator, the pager, Sony’s Walkman and the Palm Pilot in a museum. Although it’s hard to imagine the iPhone dying, change can sneak up rapidly...