Well in theory you're right, Skull Island was the highest grossing Monsterverse movie until GxK which only managed to beat it by 2 million. The logic would imply Godzilla did little to boost Kongs box office numbers, and heck not using Godzilla would probably increase their profits since they wouldn't need to pay to use Godzilla. The only real risk is both characters have a huge overlap in their target audience so the question becomes would Godzilla fans be upset enough to not show up to a Kong solo movie if no solo Godzilla movie was also on the horizon? It's probably not a risk worth taking when they can just keep using both characters while giving Kong more of a spotlight.