It's a progressive payout though. SSA pays out 90 percent of average monthly income over 35 years (approximate based on their formula. AIME) below the first "bend" at about 1200 dollars per month, 32 percent between the first and second bend, and 15% above the second bend. So SSA would get 10.6 percent of every new dollar that used to be above the cap, while that dollar would increase the person's benefit by their marginal rate (likely 32 or 15 for a high earner) divided by 420 months (you could assume the person will have a 35 year retirement to cancel out the two sides of the equation). Turning 10.6 percent into 15-32% (and most of the uncapped money would be likely to fall in the 15% if I had to guess) shouldn't be a hard thing to do for SSA with extra funds to spare when you consider that the average retiree won't live for a 35 year retirement and that there are 35 years of gains to capture between money in time vs money out time
It's a progressive payout though. SSA pays out 90 percent of average monthly income over 35 years (approximate based on their formula. AIME) below the first "bend" at about 1200 dollars per month, 32 percent between the first and second bend, and 15% above the second bend. So SSA would get 10.6 percent of every new dollar that used to be above the cap, while that dollar would increase the person's benefit by their marginal rate (likely 32 or 15 for a high earner) divided by 420 months (you could assume the person will have a 35 year retirement to cancel out the two sides of the equation). Turning 10.6 percent into 15-32% (and most of the uncapped money would be likely to fall in the 15% if I had to guess) shouldn't be a hard thing to do for SSA with extra funds to spare when you consider that the average retiree won't live for a 35 year retirement and that there are 35 years of gains to capture between money in time vs money out time