There is no benefit cap, just a progressive rate. At low lifetime contributions, you get 90% benefit compared to contribution. As you increase your contribution this moves up to marginal rates of 32 and 15%.
As long as it's still a diminishing return and includes investment income, I have no problem with that. The real issue IMO, is it's simply not diminishing enough relative to expected lifespans, and there is a huge benefit associated with being married.
In 2017, the Social Security Wage Base was $127,200 and the Social Security tax rate was 6.20% paid by the employee and 6.20% paid by the employer.[1] A person with $10,000 of gross income had $620.00 withheld as Social Security tax from his check and the employer sent an additional $620.00. A person with $130,000 of gross income in 2017 incurred Social Security tax of $7,886.40 (resulting in an effective rate of approximately 6.07% - the rate was lower because the income was more than the 2017 "wage base", see below), with $7,886.40 paid by the employer. A person who earned a million dollars in wages paid the same $7,886.40 in Social Security tax (resulting in an effective rate of approximately 0.79%), with equivalent employer matching. In the cases of the $130k and $1m earners, each paid the same amount into the social security system, and both will take the same out of the social security system.
