I've read some of those web sites and they're insufferable. Buy a house in cash so you have no payments, shop only at Goodwill, collect rain water for drinking, and never, ever get sick, and you, too can retire at 40!!
Sure, there are people in the FI crowd trying get by on $15k a year. Not my cup of tea, but if they're happy that way I say more power to them. But the community is a spectrum -- there's also a fairly large contingent (composed mainly of doctors, engineers and other high-income workers) whose savings at early-retirement time seem to cluster around ~$3M, allowing for retirement spending of $90-100k. That buys a solid lifestyle by almost anyone's standards. Obviously, that's predicated on having a high-earning career and taking saving seriously, but it's totally doable in 10-20 years.
Do-able? Maybe. High-earning is an understatement though. If you want to reach $3M in 20 years, you'd need to save about $100K per year, assuming, say, an annually compounding 4% interest rate across your savings and investments. A lot of people who might consider themselves high-earners don't even make that much in a year, let alone are able to save it.
Even assuming you're a gambler and put all your savings into stocks and get that mythical "steady 7%" return, you need to set aside $75K every year to reach $3M in 20 years.
I'm not claiming to be the world's best saver, but I think I live frugally enough, and after 20 years into a fairly good tech career, my savings is an order of magnitude+ less than that.
I don't think a 7% interest rate is unreasonable. One of the advantages of planning on early retirement means that you can take on a lot more risk with your investments, eg. putting everything into stocks (which isn't really "gambling" -- over long periods they've only ever gone up). If the markets are totally hammered the year you want to retire, no problem -- just keep working a few more years until they recover. And saving $75k a year is also quite reasonable if you have a high income, like a doctor or well-compensated engineer. I’m only a couple years into my career and am supporting my wife and kid, but we’re on track to save a bit more than that this year.
Of course, the lifestyle choices we make aren’t for everyone, and that’s fine. But they’re also not totally crazy. We can save that much still live about as well as the median American, and a lot better than the median human.