There's no sure fire option for making sure you don't run out of money, as nothing can 100% predict the future. The best we have is the assumption that the future will look kinda like the past over long periods of time.

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.

> This did a good job of telling me not to trust it

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.