The Proteas beat Sri Lanka in Monday's dead rubber to conclude their campaign after defeat against West Indies and England had scuppered their semi-final prospects.
England chased down a 230-run target against South Africa in a high-scoring thriller in which the Proteas conceded 26 extra runs compared to four by their opponents.
The likes of Steyn, Chris Morris and Imran Tahir sent down 20 wides at Mumbai's Wankhede Stadium.
Twenty20 is sometimes a gamble. Sometimes a Chris Gayle rocks up and he wins the game for you.
"We were poor with our extras," du Plessis, who had earlier termed the tournament as a 'nightmare', said after the eight-wicket win against Sri Lanka.
"Every game, we had more wides or no-balls than opposition. That's always basic and controllable.
"Your job is to try and get the ball inside the line and if you do it less than the opposition, it generally gets five or 10 runs that you don't have to score with the bat.
"That's possibly the key area for me in the first two games when we lost. That was probably the key."
A tournament like this leaves little margin for error and South Africa were guilty of not capitalising on chances, he said.
"We played good cricket leading up to the tournament... but a World Cup is a do-or-die tournament and if you make mistakes, it's not like a series where you come back into the series.
"We made one or two mistakes against England and they punished us for it. Against West Indies, we made one or two mistakes as well."
De Villiers, Amla and Steyn are all 32 and with World Twenty20 shifting to a four-year cycle, chances of them playing the tournament look slim.
"The sad thing is that it's now four years to the next World Cup -- it's not just two," du Plessis said.
"A few players here won't be there then. That's going to be the challenge now for the squad: to try and rest some of the senior players, to see if they want to go to the next World Cup. With that you can breed some young players."
(Editing by Sudipto Ganguly)
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