I subscribe to the Get Rich Slowly RSS feed, and
this article was posted today, authored by Financial Samurai. The short version of his thesis is this:
The Trinity study was performed in the 80's, when the federal funds rate was around 5%. Now, it's at 1%. That means that corporate bonds will have a lower return, and dividends will be similarly lower. Which means that you can only rely on a 1% return, and so you need 100x spending in order to FIRE.
Of course, saving up 100x spending is a pretty steep hill to climb. FS's solution?
Go back to work. Seriously. Because "What's funny about the 4% Rule is that its proponents don't even follow the rule!" That, or invest in higher-risk things like REITs, dividend ETFs, rental property, etc., all of which will give you something like 3-4% returns.
What's interesting is there's no mention of index funds in there...