Early retirement has had a catchy rebrand. The Financial Independence, Retire Early movement—FIRE, for short—popularized over the past decade or so by such books as Your Money or Your Life by Vicki Robin and Joe Dominguez and The Simple Path to Wealth by JL Collins, as well as countless blogs—is a community of people who seek to opt out of employment in their 30s or 40s by making as much money and spending as little as possible for a few, intense years. 

The idea is to accumulate enough savings to be able to live off the passive income for the rest of your life. Advocates suggest you need to bank 25 times your planned annual spending before quitting, and then withdraw just 4% of your portfolio each year. This means saving aggressively—up to 75% of your earnings—and cutting yo...