Not sure I agree. Trinity study suggests that if you retire 30 years before death, you can safely withdraw 4% every year during retirement, regardless of economic fluctuations. If you retire super early and need 50 or 60 years, your safe withdrawal rate is probably closer to 3% or even 2%, though.

Your point may be that we should expect significant economic turmoil in the near- and medium-term, much more than "fluctuations", which may be true, or may not be. Impossible to predict the future.

Consider that the 2021 inflation rate is likely a symptom of COVID. Monthly inflation rates for the last few months of 2021 were trending downward. Hopefully we can expect inflation to get back to something approaching normal by the end of 2022. Even if it takes a couple more years, there's no reason to expect that 7% inflation is the norm going forward.

(Also consider that people who are doing FIRE certainly don't have much in treasuries. It's going to be mostly in stocks. That certainly carries other risks, of course.)