Nobody cares who has the #1 ranking in ODIs. The World Cup winners are effectively #1, whether they are or aren't the best team. England would gladly be ranked last if it meant they had a World Cup.
That's not true. When Sri Lanka won the 1996 world-cup, in the years leading up the 1999 world -cup (although their was no ranking system in cricket then) - nobody was speaking or was giving SRI a free pass as # 1 just because they won the world-cup previously.
That was because their performances had declined drastically.
Teams still have to play well consistently in series to earn that tag as best ODI team.
I didn't say that they actually have to be the best team. I'm saying that it doesn't matter if they aren't. The World Cup does more than enough to make up for that.