The funny thing is most working people will look at the spending levels required to achieve FIRE and decide they don't want to make the sacrifice. They are in a prison of their own making.
Of course it's easy to see how they arrived in this situation. There is a constant stream of propaganda telling us we need to spend in order to become successful. Without that SUV you aren't a good parent, without the big house you can't be a good partner, without the expensive holidays you must be a loser. Thank Bernays[1] for all of this.
Not many people can resist that and take a different path which is why we don't need to worry about "what if everybody does it".
[1] https://en.wikipedia.org/wiki/Edward_Bernays