This whole "he must prove himself by winning the World Cup" is such a load of bullshit it really is.
No team is a one man team. Messi has 10 other players around him and unfortunately, he cannot do a thing about them. No matter how good Messi is, if the other 10 players aren't good enough, then the team isn't going to win it. Why should the lack of quality in those other players have any influence over how good Messi is as an individual? It makes no sense.
I think Messi will easily become the best player in history. A lot of people forget that he's only 24, and if most other players are anything to go by, he hasn't even reached his peak yet, and could have another 8 or 9 years in him. He'll probably only improve further and smash every record going.
As for Argentina, they could probably actually do quite well, but they need a better manager who understands how to get the best out of Messi. Atm it looks like they're trying to get the team to play like Barca for the purposes of Messi, but they simply don't have the players for it. Argentina might push for the World Cup in the coming years but they need to get that sorted and also choose the right defenders and midfielders, which is a department they're lacking in.