I know of it, and I think it's optimistic. You have to factor in the chance for inflation to go up and general equity returns to go down. I think 7% nominal returns is an outlier in the history of the world.

For example, the 30 year TIPS return at the moment is ~1% - I would consider that a truly "safe, real interest rate" return. By that standard, taking out 4% in real dollars has a significant chance of depleting your principal over time.