Felix (and Cameron) recently interviewed Bengen (who came up with the rule) on their Rational Reminder podcast, and it may actually be 4.5% now:
* https://rationalreminder.ca/podcast/135
* https://www.youtube.com/watch?v=_nYTrCxluaY
In 2006 he had it at 4.5% and in October 2020 at 5%:
* https://www.fa-mag.com/news/choosing-the-highest-safe-withdr...
The reason being is inflation.
The really high inflation of the 1970s (and 1980s) chewed up savings for retirees and brought down SAFEMAX down, but in the 1950s and '60s it was actually 5% per Bengen.
Nonetheless, it’s a good idea. Instead of cash, however, I would opt for paid part-time work if possible.
A large cash reserve isn’t protected against inflation nor will it grow; the downsides are too big imho.
Are you asserting interest rates can only go up from zero? Or only down? And for either direction, why?
Although, more than cash, a few loyal and useful kids would be a much better play to mitigate those risks.