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.