Don't quite agree. As a Brasilian myself I feel like most of the rivalry is due to Brasilian and Argentinian clubs playing each other in the Libertadores da America competition. And since we are neighboring countries, we see a ton of rivalry because of that specific tournament. There is also some extra rivalry in the mix because of the "pele vs maradona" goat discussion but it's widely accepted that Pele wins.
If Argentina was to win against the Germachine, I don't think any of that would happen. You would surely see many Argentinian fans possibly dying or being seriously hurt after provoking Brasilian fans... But that is common sense lol
Now if it was Brasil vs Argentina in the finals.. That would be a different story. But Brasil didn't get to the finals. Plus I know a shit ton of Brasilians who love messi and Barcelona so they are actually rooting for Argentina at this point.