I think that sooner or later a lot of people who are bravely going into 100% US stocks are going to be slaughtered.
Not because it's a bad investment. At least if your time horizon is sufficiently long, and you have enough discipline to stay the course. But because the truth is that everyone can say they have a time horizon of "forever" and they will be fully disciplined. But what do you trust more, what people say they will do in the future, or what has actually happened every time there has been a serious drop in the market in the past?
With 100% stock, sooner or later the portfolio will get slaughtered. Fact
But why such a strong degree of confidence that people will make self-defeating investment decisions, selling at the bottom in an emotional panic?
Especially considering that by the time somebody has saved enough to retire young, they already have a long history of making wise choices and displayed a good command of investing best practices?
The answer to your first question is because that is what has happened historically. Pretty much every time there is a panic.
Your point about the degree of self-control being better in this sub-culture is a good one. But this sub-culture consists not only of people who have successfully made the jump to financial independence (such as yourself), and thus actually demonstrated a proven capability of carrying out a long-term financial plan, but also those who are only aspirational towards this goal (such as myself).
By definition, most of this aspirational group (since they haven't reached financial independence yet despite adopting FIRE principles) will only have been saving and investing in a FIRE way for a few years, coinciding with this long bull market. Everyone in a bull market (not just the FIRE community) thinks they will be the super-disciplined group that stays the course. And some will be. But most won't.
I have no doubt the percentage of people who can do it in this community will be higher than that in the general population. But I think it's naive to imagine that somehow this is the time that has forever ended the human tendency to sell out in market panics.
For if that happened, there wouldn't be market panics in the first place. And if there weren't any market panics, there would be no equity risk premium, and stocks would deliver no excess real returns. And we wouldn't want to be in that world, right? The 4% rule (or any other projections, even those much more conservative) would go out the window.
Thankfully, I don't think that's going to be the case at all. It seems much more prudent to assume that human tendencies that have repeated throughout history will continue to persist.