I have heard that if you are retiring relatively early, 40s or so, to actually shoot for having about 30X your annual spending in retirement. So that would be a 3.3% rule. I think like others have said, you should execute flexibility when it comes to withdrawals rates.
It's more that the 4% rule is just a starting point for everyone, and each person has to assess what risk hedges they're most comfortable with.
The risks with retiring very early have much, much less to do with long term market performance, and a lot more to do with the ability to project your future financial needs.
Not only do you not know what personal factors could influence your future spending, you don't know what local economic factors could come into play.
So saving more is one hedge that can help, having a more flexible budget is another, maintaining then ability to generate income is another. There are numerous ways one can hedge against the risks inherent in longer retirement.
For some folks it makes more sense to just stay in their job longer and build a bigger 'stache. For others that's a terrible idea because they want out early for a reason, and for them, it makes more sense to do more what MMM did, to leave their career as soon as possible and work on cultivating enjoyable projects that are sustainably profitable for years.
Also, 3.3% WR only tells part of the story. What is that even based on?? I guarantee you that there are some folks with a 4% WR who are at far less long term risk than some folks at a 3.3% WR.
If I give you 3 scenarios, which would you rather?
A: A 'stache of 900K with an annual WR of 3.3% for an annual spend of 30K for you and your spouse. You achieve this by renting in a low-income co-op where all residents perform maintenance tasks as part of their tenancy agreement. You are both avid gardeners, so you work the communal garden. You had both worked in sales jobs, so you have no professional skills aside from general sales skills to fall back on.
B: A 'stache of 2.5M with an annual WR of 4% for an annual spend of 100K. You also have a 1.3M paid off house with very high property taxes and utilities. You outsource a lot of domestic labour, have country club memberships, etc. A lot of expenses that could be cut if necessary. You were an executive and your spouse an admin at the same company. Your professional knowledge and connections are rapidly becoming outdated and ageism will quickly make it hard for you to re-enter your industry.
C: A 'stache of 1.125M with an annual WR of 4% for an annual spend of 45K. You also have a 250K, paid off house with low property taxes and utilities. You retired because your partner had cancer, and it was a long, brutal battle, but they're in remission now. Still, you want to focus on quality of life and neither of you were great at that when you were both working full time. You are both licensed psychiatrists, you maintain your credentials and you stay on top of continuing education and networking. You also both do volunteer work on a very part time basis. At any point, one of both of you could pick up part time work and generate 6 figures working remotely. But right now, you both value living in your little, remote cabin in the woods, living a really peaceful, nature-focused lifestyle after years of grinding and then fighting cancer, and you may never want anything else.
As you can see, the couple with the lowest WR are actually the most susceptible to risk. Because WR alone tells only an extremely narrow sliver of the overall risk story for real human beings.
It would be lovely if there were some simple numbers we could achieve and think "yay, we're safe now," but that's just not the way it works.
I personally don't feel comfortable unless I maintain skills that I can lean on to generate income if needed. So that's my preferred hedge. In my particular circumstances, that's a lot more favourable to front-loading savings as much as possible.