Two years ago, the founders of Airbnb were asking themselves what the company could become, now that its vision of becoming the world’s largest home-share community had come true beyond their wildest expectations. That’s when they happened across a list of the top 10 tech companies of the 1990s. They were stunned—and scared. Nine of those once-hot companies were now floundering or dead, and they had all done everything right. But by simply focusing on…