This is what terrifies me about the FIRE movement.
The ~4% SWR for FIRE assumes that we'll see sustained growth in equities markets, because that's what we've seen before. But the data really only goes back to the early 20th century (the data from the "Trinity Study" looked at the period from 1925 to 1995).
I agree with you that people in the FIRE camp will still be far better off than the average person. But to me the terrifying scenario is retiring at 30 and then running out of money at 60 because the 4% rule didn't work out.
It definitely is justification for those people to make sure they build in a buffer though since its unlikely they'll be able to immediately return to their peak salary before retirement.
I honestly can't think of anything fundamental. Mobile and single page web applications, for example, became popular in that time, but those only brought back the same patterns we were already accustomed to on the desktop, not some brand new paradigm that someone from 15 years ago could have never imagined and cannot contemplate now.
Perhaps you might have to spend a week or two catching up on the specific keywords used by the popular framework du jour, but that's not a hurdle in any meaningful sense.
Granted, someone from 15 years ago coming back to an "Agile" shop will run right back where they came from, as fast as they can, wondering what on earth we are thinking the whole way.
I would argue that the real nightmare scenario is that the rules of the game or government policy are radically changed by fiat. For example, effectively nationalizing private retirement accounts in some fashion to backstop public pension systems like Social Security (which has been floated in the US at various times). A lot of FIRE plans are predicated on retirement accounts existing in something like their current legal form over the next several decades. I am skeptical.
You will do better than people who own nothing but your wealth won't last forever.
Being able to count on your own labor and having skills in industries related to fundamental survival are key in hyperinflationary economies. You can't count on the financial system remaining intact in any usable form. Most FIRE plans are about leaving the labor force to live off of returns of capital, though.
FIRE presupposes having non-liquid asset base, such as actually owning the place you live in, means of transport and potentially the tools you use, first.