Thanks for bringing this up! In my general aerospace engineering class, taught by several active engineers and project managers from JPL, one of the professors

Thanks for bringing this up! In my general aerospace engineering class, taught by several active engineers and project managers from JPL, one of the professors did a rough estimate and figured that NASA could double the number of launches without effecting budget or success rate. He thinks that, save for a few critical infrastructure missions, the diminishing returns get to such an extreme that they add no benefit to most missions. However, because NASA has to play politics for an ever shrinking pie, they can't afford to take no-brainer risks like launching ten missions instead of five because four failures looks worse than two, even if they both represent 40% failure rates.