It could easily be that B and C students are overrepresented amongst "the elites" but grades are still a good predictor of future income. Like, there just aren't that many billionaires, even if like half of them were C students I doubt it would meaningfully affect the median income of C students nationwide.

In fact I would very much expect the grades vs income distribution to be much flatter for the upper classes simply because their income is largely based on "already being rich, powerful, and connected" rather than succeeding in a pseudo-meritocratic competition.