"Humans believe they are more intelligent than dolphins because they have achieved so much - New York, the wheel, wars, and so on - while all the dolphins ever did was mess around in the water having a good time. Conversely, dolphins believe that they are more intelligent than humans for precisely the same reason." - Douglas Adams
Conversely, only humans have the ability to wipe all forms of intelligent life off this planet and hasten their own destruction.
Furthermore, you could've made the same comment about us [humans] just a few thousand years ago. And considering the rate of evolution compared to the length of time this planet is going to be around for, it's probably a bad idea to extrapolate the fate of dolphins in the future based on their current capabilities.