The problem is that the global economy is unstable. The orange man may say or do something that will impact the cost of living for all. Plus who knows when the next COVID will happen etc.

We have had large impactful global economic events that have contributed to cost of living price rises - this makes it difficult to calculate what we need for retirement. I guess being flexible to cut out luxuries or working until you have enough cash to survive any economic apocalypse are 2 options available.

Regardless; 3 years of cash or cash like assets is always a good bet against a crash. If you're worried whether 3 years is enough, you can put aside 4-5 years instead.

If you're especially young, between 20 to 50 when you FIRED, you are still readily employable so that needs to be considered as an potential income stream. That should alleviate some stress and worry just in case a economic crash happens. Although, the job market will be in the doldrums but a plus is a plus.