Cornerback is one of the most difficult positions to play in football. With the new(ish) rules regarding player contact beyond 5 yards of the line of scrimmage specifically put in place to help generate more offense and penalizing physical cornerback play, it's becoming increasingly more difficult for cornerbacks to be reliable in coverage. This is especially true in man to man coverage, which the Patriots (and Revis) play a lot of.
Revis, on most plays, is assigned to cover the best wide receiver on the field for the other team. He's very physical within that 5 yard range and is fantastic at throwing people off their routes, and then has an uncanny knack at gracefully following the receiver step for step and covering them completely. A shut-down corner, like Revis, will completely shut down "one half" of the entire field, basically removing that player from the game. When a team has their best receiver neutralized like Revis can do, it seriously harms their ability to run their offense.
Free safeties, like McCourty, basically play back and stay behind the offense, no matter what. They act like the last line of defense against the offense. He usually will be covering his zone (in cover 2, where two safeties drop back) or playing "center field," which means he's playing the back middle and is responsible for the back end by himself. While most free safeties are smaller and faster than strong safeties, which play on the strong side (where the tight end lines up), McCourty is a fantastic combo of size and speed. Inherently, the free safety position is considered "easier" than cornerback, where you stick to your zone in passing plays, provide that last line of defense, and come up to make saving tackles in the running game.
Even though Revis is basically covering one player vs a whole side of the field, if he's effective, it has a MUCH more impactful effect on the offense as it allows the safety on his side to cover other players, as well as removing what may be their best player from contention. In addition to all of this, the drop off from McCourty to their second best free safety or even someone lesser in free agency is much less than Revis to another player. He's perhaps the best cover guy in the league, at a more important position. This isn't to say that free safety isn't important, of course it is. But a shut down corner like Revis can change a whole defensive (Patriots 2014 vs Patriots 2013, Super Bowl victory vs. AFC championship loss).