> If that worked, everyone would do it, and so it would no longer work.

This simply isn't true. Sometimes there are dollar bills on the ground. It takes a lot of years for everyone to pick them all up.

Keep in mind that the efficient market hypothesis disproves(TM) starting a successful business just as well as it disproves finding a successful trading strategy. ie: if that were a good startup idea, someone would have already started it, so it can't be an opportunity any longer. EMH is a useful tool but in reality it takes a long time after a fundamental shift creates an opportunity for it to be arbitraged away, and sometimes the opportunity ends due to another fundamental shift, not due to people arbitraging it away.