Firstly, if you're playing for a nation which never has any chance to win the World Cup, then of course you shouldn't be judged on this basis (i.e. my inclusion of George Best in the top 10).
However, if you're playing for a major nation, and want to be considered the GOAT of the game, then in my opinion you do have to lift the trophy, preferably while dominating the tournament. It might only occur every four years, but it's long been considered the pinnacle of the sport (I do accept that in recent decades, the club game / Champions League has grown in its relative importance vs. the World Cup). It's a similar argument to the Olympics - only held once every four years, but it's the place where legends are made or broken.
I think a direct comparison between Maradona and Messi is instructive here: both played in middling Argentina teams, but whereas Maradona almost single-handedly (pun intended) drove his team to the title in 1986 - scoring wonder goals against both England and Belgium, plus setting up the winner in the final - Messi was distinctly average in 2014. He was far from inspirational in Argentina's run to the final, and was effectively cancelled out by the German defence in that game.
Of course, Messi's stats at club level are almost incomparable (although Di Stefano does still have one more EC/CL title than him), and his consistency at the very top level is perhaps unparalleled - certainly not matched by Maradona, although it may have been by Pele. However, to claim that he's the football GOAT when he can go missing in his nation's biggest game in a generation is something which simply doesn't wash with me.
