Except for the most part you can't replace a player with an average one. WAR allows you to see what value will be lost on your team if a player should go down and you have to grab a AAAA player to replace him. They've gotten pretty good at defining replacement level anyway.
Who cares if you can replace the player with an average one? You don't need to create and artificial Replacement Player to judge the value of a major leaguer. In fact by judging them against fellow major leaguers you get a better benchmark. It is much more accurate to say Adam LaRoche had an OPS of .735 while the league average was .773 for all firstbasemen. While OPS wouldn't be used in reformulating WAR, the idea is use cleaner data.
