In 2008, a 23-year-old Sam Altman stood on Steve Jobs's stage demoing a location-sharing app called Loopt. It raised over $30 million and sold for $43 million. Life360 does the same thing today with 121 million users and a market cap around $3.5 billion. So what killed Loopt? The phone he was on stage to launch. The 2008 iPhone banned apps from running in the background. Apple didn't ship background location until iOS 4 in 2010. Loopt's entire promise was a live map of your friends, and every…