x-ray,
If it was as simple as you suggest, i.e. choosing the group favourites, then there would be little point of the other 24 teams showing up. The 8 you suggest would all have to win their groups, plus their last 16 games before landing up in the quarter finals - a tall order - all it would take is one to be runner up in their group, or to slip up in the last 16, and unfortunately your theory would go up in smoke.
For example, the likes of France, Italy and Germany are no longer the forces they all were over the past decade and there are very good teams up against them like Mexico, Uruguay, Serbia and Paraguay, any of whom could upset the applecart.
Holland, too, have a difficult group, but for me are the dark horses of this WC, and I can see them doing quite well, although they'd probably come up against Brazil if they made it as far as the QF - a daunting task?