This is a very important point - we need to distinguish between the current real rate of return and the expected real rate.
The average rate of inflation over the very long term has tended to average around 3%. However, the current rate of inflation as "The Ghoul" correctly notes is close to zero.
We know that over long periods of time in the USA, UK and other countries where we have good data that the gross return on deposits (or proxies for deposits such as 3 month money market rates) have tended to offer a real rate of return of around 0% to 1%pa before taxes.
Logically therefore if, all else being equal, taxes go up, the expected return on cash deposits has to go down relative to the average over time since the after tax return is expected to be lower.