It wouldn't be for some people. There's a list of criteria. First it was that he couldn't score against English teams. Then it was he couldn't score in England. It was also said he couldn't score a header with a top hat on before the CL Final in 09
Then there's the xaviesta thing, which is such a lazy argument I don't really take seriously. It wouldn't inevitably be debunked when those too ease out of the side.
Each one will be ticked off the Argentine's list, but if he doesn't win the world cup it will always be mentioned in the best player ever debates.
If Messi doesn't win the world cup but keeps this up, I believe when he retires it would possibly look like 55% in favour of Maradona as the best and 45% in favour of Messi.