Actually, I pulled the numbers off the historical Social Security document that I referenced earlier and created a spreadsheet. I started from 1966 and went through 2005 (forty years) and used the assumption that for every year worked you maxed out the contribution amount, and I used the actual S&P market returns for those years. The results will probably surprise you:
Forty years of the employee-only contribution totals: $900,339.14
Forty years of the self-employed contribution totals: $1,460,193.64
Let's pretend that you were able to find a guaranteed return that held constant for each of those forty years. What would the "modest interest" rate need to be in order for you to have "several millions?" (I used $2,000,000 as the target):
Forty years of employee-only contributions: 15.67%
Forty years of the self-employed contribution: 13.49%
Getting that kind of return year-in and year-out is pretty unlikely. And keep in mind that I assumed you maxed out your contributions each and every year for those forty years.
Summary: Sure you got screwed by the Social Security scam, but not as bad as you think. Pity the 25 year-old who's return can't possibly be in the positive territory.
Just a reminder...I said that it should include both the employee and the employer contribution, because both come out of my cost to the employer for me as an employee. That would halve the interest rate for forty years of employee-only contributions: 15.67% (your figures) and that is reasonable for most of those years. So that require an interest rate of 7.84% to net a return of $900,000. I earned at or exceeded the max for 40 years, btw.
The 'screwing' is due more to corrupt politicians than interest rates. If the original stated plan had been followed honorably, the money in the trust at this point would probably be incalculable.