In chess, the Queen is the most powerful piece, but the game can still continue when removed from the board, while the King being outmaneuvered ends the game. Selene is the most powerful member of the Inner Circle bar none, but she more or less joined for the funsies and doesn't push the organization towards its goal unless it amuses her; if she was removed she'd just be replaced. Shaw is the one who reorganized them to do something other than kinky BDSM and start moving to rule, if he was removed and not replaced by someone with similar vision, the group would fall apart.