Big difference between Rex and McD: the players and owners have bought into McD's plan. Rex was clueless and it was obvious by the end of his second year that he'd lost the team.
Why do the players (and the owners) believe in McD and not Rex? Rex took a top-five defense and decimated it. McD took roughly the same players and rode that defense to the playoffs, in spite of having an anemic offense with a gun-shy QB. Then he (and Beane) dumped the dead weight, cleared cap space, drafted what looks to be a competent QB, bolstered the o-line (on paper, at least), improved the WR corps, and still have ten draft picks to spend. Barring a total meltdown, they get at least two more years, and probably three.