NFL free agency officially opens on March 12, presenting an opportunity for each team to improve their roster. But every team also has the chance to get worse -- if their best players choose to walk in free agency.Losing a top free agent can be catastrophic to a team's success. For example, the loss of RB Josh Jacobs in free agency turned a middling Raiders team into one of the worst in the NFL. And for NFL teams, keeping their top free agents is just as important as acquiring new players.Let's ...