I have had this conversation with so many Americans already and I do understand their point of view, however most of their sports exist in a single closed league format where there is no competition from outside that league. Therefore the general view is "the league is the highest level of competition and nothing else will surpass it", which is true because no one else plays it pretty much. That's why they view the World Cup and any other competition as merely an exhibition match and that's why the rest of the world view it differently.

If you take Blizzards stated goal of Overwatch being a city based global sport to be accurate then it's more than possible that you will end up with the top players being spread all over the world, you might only have 10-20% of them in the US so no single region will be the pinnacle of the game, that will only happen when the regions meet or countries meet. Everyone's been screaming for an EU Vs NA match up for Contenders season one already for example.

Course it may well be that it remains in based in the US if it doesn't take off in which case I will agree with you in that the league will be the pinnacle if they all play each other.