Yes this analogy would work if every brain became a cancerous growth that realigned laws and basic pathways to serve the survival and growth of the brain, but this is not the case, the brain in the body has a maximum size (proportionality) and also serves the rest of the body. Not only the brain's satisfaction, but the skin, the gut, the sense organs all rely on the brain however the brain serves them primarily before serving "itself"

An organism's reproductive gain factor isn't that sensible a metric of its success. A better one is how adaptable it is.

Because of our brains, humans are extremely adaptable. Even with primitive technology we spread throughout the globe, and unlike wild cats or bears we haven't needed to speciate into each climate, develop camouflage fur, etc. The biological investment in an advanced brain gave us the opportunity to thrive everywhere.