What do you mean? The retirement planning with Monte-Carlo gave a better result than the base assumption of 4% returns and 2% inflation. The information MC provides let's you, when taking into account your risk threshold, whether you're satisfied with your savings or not by giving you more realistic odds of how long the money will last.

If you acknowledge that assumption, then you have to choose what model of the historical market you want to use a predictor for the future.

You can use exact market data, test retiring out at every single possible year at every possible asset allocation and see what happens. This is the basis of the 4% rule from Bill Bingham.

You can pull random years out of history and string them together a bunch of times and see what happens with a given asset allocation. This is a naive Monte Carlo simulation.

You can pull blocks of years and string them together, to acknowledge that there are repeating trends in years (what happens in one years influences the next) and do a Monte Carlo simulation which is a bit less naive.

There are more esoteric models overly smart people have constructed, each of which will give you slightly different answers for a safe withdraw rate.

At the end of the day though we are all just crossing our fingers, and hoping for a black swan catastrophe to not happen.

Usually critics of these estimates say stuff like “the simulation includes scenarios like the Great Depression”.

They give you a range of results that illustrate things like sequence of return risk, which most people have difficulty understanding and incorporating into plans. Many people have an underlying “stonks go up” assumption that may not serve them well.

I don’t have reason to believe that 40 years in a row would all have randomly and independently chosen results from a distribution (pure Monte Carlo) as a better model than past actual results.