Once again it seems to me like we are seeing arguments against something that no one, or almost no one actually does. Who here at 25 or 35 made concrete decisions about what the rest of their life would look like, then retired with no ability to change that vision or plan for doing so? Just like almost no one says "4%, baby!!1!11!" and blindly withdraws and spends their 4% + inflation number, I don't think anyone ( with "anyone" being slightly hyperbolic, but only very slightly) says, at 30 years old, I know exactly what I want until the day I die, I will budget for that and only that, and I will never change anything".
Yes, they may leave their desk job at 30 and live off investments. That doesn't actually preclude deciding they want to live in Spain when that was never the vision at 30, or having a kid, or getting married. (And of course while they decide to have a kid which increases costs, they may also decide to live in a can, which decreases costs. Or take in foreign college students because they find it interesting and it also happens to make them some money. Or no longer own pets. Or get really into gardening and not only reduce their produce bills to almost nothing, but also have eggs, some of which they sell to neighbors to cover the cost of chicken feed. There always seems to be the assumption, from the doomsayers, that future changes will end up costing more, and that there won't be offsets, either natural or as willing sacrifices, with things that end up costing less.
And of course there's also the fact that people can go back to work if all else fails. And yes, they may make significantly less, but to them, that risk is worth the gamble when it most likely means they get to stop working full time before 65, or even 55. If someone stops working at 30, it seems to me like they already have a decent idea on whether they will want children. It's not as though they are making that choice at age 12 when they have no sense at all of what life during child-bearing years look like. (Admitedly, this is more true for women, since mean can easily decide to have kids at 65, and for most women that's not viable.) But suppose three years after FIRE they meet the love of their life (or have a broken condom) and decide they want kids after all. Okay. Maybe they return to work. And maybe that 3 year gap means a 20% decrease in pay. Well, life has changed and they go back to work.
The argument seems to be that 30 year old (or pick a FIRE age) you doesn't know what 50 and 60 and 80 year old you wants. Well, of course that's true! But for smart, resourseful, thoughtful people, and especially those not retiring on the leanest of budgets, I don't see how that's problematic.
I don't know of anyone who thinks that nothing about their plan or situation or desires can or will change, and that they can just blindly live on 30k forever, with exactly the same expense patterns, just because some FIRE movement tells the that.