Marginal impact decreases, but the absolute impact is still positive... And the marginal impact is still quite high at the 85th percentile of income because the income distribution is very skewed (the absolute dollar difference from 85th percentile to 99th percentile, where most HN / YC founders probably are on the intelligence scale, is probably much greater than the absolute dollar difference from 15th to 29th percentile, as an example).
