Given how well McCaw has played in the RC, I don't see any indication that he's on his last legs physically - he has probably been the best player in any team on show. He is still far and away the best player on the planet. So anyone thinking that he is somehow on the steep downhill slope to obscurity is kidding themselves. What I see him doing is trying to keep that for another four years by having some long service leave next year and coming back fresh and ready. Its not what we would expect from ordinary players but McCaw is no ordinary player.
No one knows if it will work but I reckon if he hadn't chosen to do it then he would be very unlikely to make it to RWC2015, just because of the wear and tear on his body. As an Aussie I wish it wasn't happening, but anyone who thinks its the beginning of the end and looks on it as a good sign for Aussie rugby is dreaming I tell ya, dreaming.