It certainly isn't. World cup history shows that either the favourites win or the winning team play some very good cricket. All this rubbish that unlike Test cricket the one dayers don't prove that much.
World cup 1979 West Indies win- favourites.
WC 1983 WI favourites reach the final.
Australia 1999 and 2003 favourites win the tournament.
All the other winners although not starting out as favourites went on to play great cricket.
India 1983 was my favourite shocked everyone with some great tactical cricket and really played as a team. With of course the genius of Kapil Dev. If I remember rightly the great Gavaskar did very little it was the unsung who played out of their skins.

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"Checkout the big brain on Brett" Pulp Fiction