The 4% Rule was never "100% safe" nor "failure-proof". It's a very conservative way to approach a 30 year retirement plan and for most people who haunt these forums and are working towards a portfolio that'll be 25x their planned annual spending it will form just a part of their financial plan. Almost nobody will take out 4% + inflation every year like a robot. Almost nobody will never receive another dollar of income [PT job, pension, gov't benefits, inheritance, home sale, etc...]. A retiree who starts out with 25x their planned annual spend and has some spending flexibility and some additional income potential is going to have a very low chance of running out of money over a 30 year retirement.
No plan is perfect and getting to 0% failures in a simulation does not mean you have a plan that cannot fail. Even if you don't run out of money there are many ways to fail at FIRE that are not financial. Serious health problems, relationships ending, being miserable, etc... So having a solid financial plan is important, but it should only be a part of your overall FIRE plan which needs to address all the other dimensions of a successful happy retired life. Life is a limited time opportunity and there are marked declines in several key areas as we age so you cannot continue to trade life force for money indefinitely without paying a price even if you end up with an insanely low % withdrawal rate.
I've mentioned 30 years above a couple times because it ties in nicely with the Trinity Study. If you are going to have a much longer retirement [mine could be ~50 years based on my parents being in their 90's and going strong] you need to address that in your plans. To some degree a 30 year retirement and a 50 year retirement can be funded from a similar pool of investments, but obviously the longer retirement is not as robust due to the extended requirement to produce income. You can deal with that by saving more to get to a 3.5%WR or a 3%WR. You can also stick with 4%WR and use PT work, Gov't benefits, expected inheritance, more spending flexibility or the future sale of a high value home to reduce the longevity risks.
I was planning on FIREing this year and I am still on-track to do so. I hit 4%WR earlier in the year and am now up to ~4.5%WR. I'll do a number of things to reduce the pressure on my portfolio initially so it can recover back to 4%WR. As I get back to 4%WR I'll relax my additional measures and return to my "normal" FIRE plan/spend. Despite the COVID-19 crash/uncertainty a retirement based around the 4% Rule seems like a reasonably safe way to approach things and a good trade off when you consider all the non-financial factors that have a huge impact on the success of our retirements.
