I think that you have to come at this from an analytical direction rather than an empirical one; the problem is that everyone can say that the approaches that you have used to show how difficult it is to model the world cup are just the wrong ones and you need to redo the experiment.
Analytically the difference between premier league and the world cup is that you have momentum and continuity in the premier league and the world cup is essentially one shot. So in the PL team A will play team E and G and H before it plays team B, team B may play team E and H and Q (which played G). Team A may be winning games that your strength model shows they should lose, Team B may be losing games... and so on and so on. There is more evidence that might matter. More importantly you can be wrong quite a lot of the time in a season and still be right at the end of it (as the bounces of the ball even out over time). Not so much in the world cup - one goal knocks you out and there is no coming back! Basically the world cup demands an algorithm that works with less evidence and with a much higher degree of accuracy.
Analytically the difference between premier league and the world cup is that you have momentum and continuity in the premier league and the world cup is essentially one shot. So in the PL team A will play team E and G and H before it plays team B, team B may play team E and H and Q (which played G). Team A may be winning games that your strength model shows they should lose, Team B may be losing games... and so on and so on. There is more evidence that might matter. More importantly you can be wrong quite a lot of the time in a season and still be right at the end of it (as the bounces of the ball even out over time). Not so much in the world cup - one goal knocks you out and there is no coming back! Basically the world cup demands an algorithm that works with less evidence and with a much higher degree of accuracy.