Taking the RE out of FIRE
Retiring early, like really early, like 35 is a pretty bad idea for a bunch of reasons. Financial independence on the other hand, as soon as possible is a great idea for a bunch of reasons.
1. Investment portfolio sustainability
The odds of a sub-$2 million portfolio lasting for 60 years, when being drawn upon are pretty low. That is just too many years of occasional large declines, one or two big mistakes, a lifetime of random, one-off, unbedgetable budget busting unexpected expenses or maybe even illness.
2. No social security
You don't need that many years to qualify for Social Security but you do need a lot of years to have Social Security be meaningful. It is calculated based on your highest 35 years of earnings. If you retire at 35 or 40 then your calculated payout would include a lot of zeroed out years. You might think there won't be Social Security when your time comes. That could be the case but that outcome seems unlikely but years of no earnings ensures your benefit will be minimal. Reduced Social Security is a more likely outcome.
3. Not being challenged can increase the chance of cognitive impairment
No, I don't have data but my opinion is that intellect and problem solving skills are use them or lose them attributes.
4. Financial independence on the other hand...
... doesn't have to mean punching a clock at a job you don't enjoy wishing the week away to get to the weekend. Some FIRE how-to's give suggestions on how to hustle to create a series of side gigs which lead to multiple income streams. Quite the opposite of being retired, that sort of hustling is hard work, harder than mindlessly punching a clock somewhere and maintaining that sort of personal empire is plenty challenging. Additionally, that sort of self-employment income will count toward your Social Security calculation. Finally, your odds of portfolio sustainability go way up.
5. Own your time, set your schedule
Owning your time is freedom, literally it is freedom. If you can figure out how to have that freedom and derive an income from your idea of freedom, you'd be foolish not to.
6. Live below your means
This is a fast track to FI, to financial independence. Paraphrasing Naval Ravikant, people who live below their means have a freedom that others can only dream about.
7. Figure it out now or figure it out later...
...but you'll be happier and have the life you want if you can figure it out now. My wife and I are incredibly lucky to have figured this out many years ago.