Very few people will blindly spend exactly the same 4% for 30 years, let alone 40 or 50 years.

Simulation models get less accurate the more fuzziness there is about the reliability of their assumptions. There's enough fuzziness in the normal 30-year simulation; anyone that thinks they can remotely predict their spending 50 years out is... uh, crazy.

Not "very few", absolutely no one will spend exactly X%+projected inflation year over year. No one has ever spent so precisely, it doesn't make any sense, and yet it's the cornerstone assumption of the calculation.

Life will simply never cost the same amount (+projected inflation) every single year. It's an insane assumption in terms of precision prediction of the future, but it's a great assumption for looking at broad strokes impact of decision making in the present.

Plus the "projected spend" is a very rough estimate, often including significant fat and padding.

Fat= optional spending line items that could be easily trimmed, such as travel, restaurants, subscriptions, memberships, collectibles, luxury services, etc.

Padding= an added margin to projected spending just to be safe (rounding up error), which effectively lowers WR without calculating it as a lower WR, which totally throws off the entire output of the calculation.

The calculation is based off of completely unrealistic assumptions and entirely made up inputs. Person A could input unrealistically low spending numbers and get 100% output, while person B could have a ton of rounding up error in their inputs, half of their budget allocated to fat, and get an 85% output from the calculator.

100% success is a comforting illusion, it's not a prediction of safety.