The point of the article is that there is a great degree of uncertainty regarding what a "safe" withdrawal rate is, and experts disagree on what rate may be safe or what system to use to plan your spending going forward. Thus, the discussion of how 4% may not be OK, but under other prediction systems a higher number may even be safe.
The takeaway being, according to the article, that you should sit down with an advisor before retiring to plan out a strategy. Of course, my counter to that is that *nobody* can effectively plan a safe withdrawal rate, even someone with financial expertise. Only with a crystal ball can you really predict what will end up being safe.
To me, it's fairly obvious that if you are retiring very early and need your portfolio to last 40, 50, or more years, that retiring in today's very low-rate and high-valuation environment, one should be quite conservative with spending. What exactly is conservative can't be quantified, but my own guideline is to use 3% vs the common 4%.
But at the end of the day, the one thing that can't be easily built into these formulas is that people's expenses can be more or less flexible depending on the nature of the expenses. Someone retiring with $500K and expecting to spend $20K/yr probably can't cut much of anything out of that budget if their investment returns suffer. Someone retiring with $5M and expecting to spend $200K/yr can probably cut their spending by 20-30% if necessary.
The point being, "SWR" is an individual thing, depending on how long you need your portfolio to last, how conservative you want to be, and also how flexible your expenses are. The whole idea of finding the one right SWR for everyone is not practical.
