You think it’s hard getting to the top? Try staying there. The accelerated churn rate of the S&P 500 indicates that at least half of today’s top U.S. companies will get replaced by someone new over the next decade. That is a mind-boggling market value of $13.5 trillion up for grabs. And the craziest part is who replaces the old market leaders: It’s often companies that, just a few years before, were considered scrappy little startups.