You're forgetting that plenty of intelligent people don't work to maintain their intelligence, which means their intelligence will flatline or even decline.
To take the gym metaphor you used, some people are naturally athletic and some are not. A non-athletic person can work out to improve their fitness level, and a naturally athletic person can eat pizza and smoke a pack of cigarettes every day. In not too much time at all they will have traded places as far as their athletic ability goes.
To take the gym metaphor you used, some people are naturally athletic and some are not. A non-athletic person can work out to improve their fitness level, and a naturally athletic person can eat pizza and smoke a pack of cigarettes every day. In not too much time at all they will have traded places as far as their athletic ability goes.