The 4% rule accounts for inflation. The stock market on an average gives you 7% a year in returns adjusted for inflation. The 4% gives you enough cushion in case of economic turmoil.
The stock market on average has given you 7%. That may not be a safe assumption going forward - going far forward. I might live another 30 years. If you FIRE, you might live another 50. Will the stock market continue to yield 7% over the next 50 years? That's a larger assumption than I'm comfortable making.
Why is it such a crazy assumption? Going back to like 1870 a $SPY equivalent has yielded something like 8-10% on average if you reinvest dividends. In that time there have been two world wars, a handful of pandemics, election turmoil (oversea and aboard), many bubbles in tech and housing, and yet the long term average was 8%+.
FWIW there have always been and will always be spelling financial doom right around the corner, but betting that way has been a failing strategy save for lucky few who got the timing just right.
Let's say you didn't invest in the US stock market, because you lived in, say, Japan. How did the Japanese stock market do since 1870? Pretty well, except for that part in 1945 when it went to approximately zero.
Ditto Germany in 1945.
Ditto Russia in 1918.
Ditto China in 1949.
Is the US going to be the US of history for the next 50 years? Or is it going to be one of Japan, Germany, China, or Russia?
Or is it just going to be, say, the UK? How did the UK stock market do over the last 50 years?
"Better be weary of making financial assumptions because you may end up on the losing side of a world war and then it will all be for naught" isn't very useful thinking imo
If you think America (or wherever you live) is going to end up like Germany in 1945, or Russia in 1918, then it won't matter in the slightest what you've invested in anyway - you plus everyone around you will be in the same (half underwater) boat.
"Ditto" for if $SPY suddenly tanks or underperforms. There are going to be much more immediate concerns for everyone if that happens.
"Accounts" wrongly, though? 4% inflation for the next half-century seems like complete fantasy. How do you imagine it possibly being sufficient to address the ballooning debt?