I think Messi was the greatest footballer of all time already, even before he won the World Cup.
This sort of thing can’t just be decided by one match or one tournament alone. What makes a player great is the fact that they can perform at the highest level consistently over a long period of time, in many different contexts. And Messi has done that better than anyone else; that’s the reason why he has broken so many records. (Though of course Cristiano Ronaldo is also up there in that regard.)
The fact that Messi has won the World Cup now just finally puts to rest the argument that Maradona, Pele, Zidane, Ronaldo Nazario etc. were better than Messi just because they won World Cups and he hadn’t. But I don’t think this was ever a valid argument to begin with, as it totally disregards everything that a player did (or didn’t do) outside of those few matches.
As Jorge Valdano quite succinctly put it: “Messi is Maradona every day”.