Its 3 weeks every 4 years. The true foundation of football is at the club level, where the likes of Ronaldo and Messi have dominated more than anyone before them.
The problem is, this is not logical. If Player X is GOAT, then Player X should be able to dominate in any setting, bar having absolutely horrid teammates. Ronaldo Lima dominated in every team he played in, whether in Spain, Italy or for his country.
Ronaldo CR7 dominated in the EPL, Spain but failed to do much for his country, but then Portugal are really awful.
Messi dominated in La Liga, nowhere else, and is a failure for his country despite having had an amazing team in 2014.
This is the biggest problem when it comes to discussing GOAT. Messi detractors rightly question the World Cup. If it was as insignificant as you said, Messi should just as well have done far better in 4 different attempts. Probability alone would dictate that he would do well 50% of the time. But 100% of the time, Messi has failed to show up at the World Cup. Questions rightly should be asked. Why can you not perform when you have teammates like Aguero, Di Maria, Higuain, Mascherano, etc.?
The ONLY plausible explanation under your theory, is that Messi simply thinks the World Cup is a sideshow and doesn't care at all. But even so, even assuming Messi doesn't care about the World Cup, for a supposed GOAT, if you are playing against teams like Bosnia or Iran, you should be expected to bang in goals or assists for fun. But for some reason, he failed to do so.
So that leads to only three logical conclusions:
1) He really, utterly does not care about the World Cup, and can't be arsed to even put in 10% effort against teams like Bosnia
2) He's the unluckiest player of all time and it's his teammates and/or coach's fault
3) While talented, much of his success at Barcelona was the result of his teammates, Gaurdiola and the team being built around him
I think options 2 and 3 are both plausible, but trying to obfuscate the issue by detracting the importance of the World Cup is nothing more than a straw man.