Just as I am not assuming future market returns will match historical returns, I am not assuming that I will receive a 2% return. I assume I will receive a zero or negative return. I already showed you how I can get 2.7% from TIPS. I know that is more than I'll ever get from Social Security.
Firstly, as I've been saying, to view what you get from social security as a rate of return on an investment is to fundamentally misunderstand the purpose of the program.
Secondly, to believe you (or Greenspan) can possibly know what that MEANINGLESS number is if you insist upon calculating it requires a forecast of future payments, benefits, inflation, etc. It is impossible to be certain what the "return" will be or whether it will or won't exceed 2.7%.