@NorthernBlitz it depends on what you mean by "skinny fire", lean FIRE usually refers to having just enough to cover your bare bones expenses, it's not exactly an amount.
For someone whose bare bones expenses are only 17K/yr, 600K may be quite a generous FI number, but it might be extremely lean for someone else.
No one advocates trying to live forever with thin margins. What people here do advocate is questioning how much you really need to live on, if your goal is to retire early.
It's really, really, really hard to save multiple millions in a short period of time. That's why so many people aim for lower numbers because the alternative is to continue working, and being a FIRE forum, a lot of people don't want to do that.
It's simple math, in order to accomplish the truly extraordinary feat of retiring very young, you must be willing to do the extraordinary.
Also, your argument about difficulty finding work in downtimes isn't valid because people who are FIREd don't need to find great lucrative jobs in downtimes, they often only need a little bit of supplemental money or they can often even just wait until the worst of it is over.
Having a large cash amount available makes that pretty easy. In addition, they're ideally positioned to retrain/respond to changing markets, because again, they don't depend on uninterrupted large income.
Now, that said, I'm not someone to scoff at a 2M number, that's the bare minimum of what I plan to save, probably more in the 3-5 range depending on what kind of work I feel like doing. But I'm not aiming to permanently leave the workforce early, and that's the big difference.
The early retiree people here tend to really know what they're doing, we don't need to worry about them. They're at a hell of a lot less risk than the really high spenders who lock in a lot of very high fixed expenses and don't have a lot of room to pivot if shit really hits the fan.
Someone who can be very comfortable spending under 20K/year may actually be far more resilient in the face of major economic disruption than someone who has a few million invested.
What I find is that people tend to look at others through their own lense. So someone who lives very comfortably on 60K/yr might not even be able to fathom how some of my colleagues are stressed out of their minds because despite saving over 300K/yr and working into their 60s, they worry they won't be able to support their lifestyles.
Likewise, that same 60K person may look at someone with a sub 20K budget and think that their level of risk and sacrifice is unsustainable.
Personally, I am infinitely more comforted by my ability to keep my fixed costs low and my capacity to generate paid work in such a broad range of markets and under almost any circumstance, It would take a lot for me to ever be at real risk, even if I only had a small stache. That reassures me in a changing world far more than the millions I'll have in the bank.
Luckily for me, I can have both, but again, that's only because I'm not actually aiming for FIRE.
Also, to address your asterisk, I don't believe anyone in this community gets fixated on an old number and then retires early on too little because they forgot to continue caring about the math. The overwhelming majority of people who actually accomplish FIRE are far more financially informed than Gen Pop.
I think that's an infinitely higher risk for people working to full retirement who never gave their savings much thought and closing in on a million think "wow, that sounds like plenty" and don't realize how constrained their retirement will be because they've never ever tracked their expenses.
ETA: this is my parents, my mom thinks her sister has a modestly comfortable retirement, and she does, she has a pension that provides her 40K/yr. What shocked my mom is that that pension was worth a million, she assumed it was about 300K because that's what she learned would provide a modestly comfortable retirement when she last looked into planning her retirement...back in the 80s.
I easily believe those people exist. I have a hard time believing many FIREes pull the trigger without thinking it to death. At least, that's what the bajillion OMY threads on here would suggest.
*Edited for grammar and clarity