The replacement rate—the percentage of pre-retirement income that Social Security covers—is highly progressive. Lower-income earners see a high replacement rate (up to 90%) because the bulk of their AIME falls into the first bracket. For high-bracket earners, whose AIME easily clears the second bend point, the marginal replacement rate on earnings in the 15% bracket is very low, yielding an average replacement rate that often drops below 25% of pre-retirement gross salary.
If you are an early retiree planning your escape, the second bend point is the critical milestone. Working long enough to cross this threshold maximizes the high-yield 32% bracket. Working past it is rarely a productive use of your final working years, as the SSA will only credit you with 15 cents on the dollar.
For a detailed analysis of this trade-off, see the analysis in Social Security & Early Retirement: Know Your Bend Points.