> As frugivorous apes we got enough vitamin C from our diet that that mutation was a survival advantage. Synthesizing nutrients you can get from your diet is a waste of energy.

This is speculative and honestly not the simplest explanation. Deleterious mutations will happen in the absence of selective pressure, and there was no selective pressure.

You don't have to cast everything as evolution playing 4D chess. Sometimes genes just break, and not for good reasons.