Polaroid. Walkman. Palm Pilot. iPhone?

As demand for Apple's signature product starts to wane, now is the time for CEO Tim Cook to find the next act

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 favourite 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 on contraptions that are deeply entrenched in American culture.
 
Consider it was as recently as the mid-1990s when I