The success of the human species can’t be attributed to intelligence alone. Bipedalism and manual dexterity are important factors in our success and their contributions can’t be isolated. Dolphins and other marine mammals are arguably intelligent, but they lack human dexterity to manipulate the environment as we do.
Another thing to consider is that the success of Homo sapiens as a species was not an inevitability, as difficult as that may be for us to imagine from our current perspective. Anatomically modern humans appeared 200,000 years ago. Behavioral modernity is believed to have appeared 50,000 years ago. Yet, through the great majority of this period, humans were a minor species of hunter gatherers, existing only as small bands of nomadic groups. It was only with the development of agriculture 10,000 years ago that human population really began to grow and achieve “success” in the evolutionary sense. Of course, intelligence was a prerequisite for domestication of other species, but the late appearance of this practice (homo sapiens existed without agriculture for 95% of its history) suggests that intelligence is not a clearly advantageous adaptation.
Of course, this is not to say intelligence did not help our species survive. But the evidence generally suggests that intelligence by itself is not advantageous, and this in turn means that intelligence is not an evolutionary inevitability. Like I mentioned, intelligent species have not necessarily been successful. To the contrary, intelligent species such as non-human primates, dolphins, and elephants have been fairly unsuccessful in terms of the numbers that currently exist. Also, keep in mind that since the Homo genus appeared over 2 million years ago, our species is the only one that survived. Neanderthals, fairly intelligent in their own right, only survived until 20-30,000 years ago.
Investing energy into breeding, rather than brain development, may be a better evolutionary strategy.