If retiree B wants to spend 4% of their current stash for 30 years they have a 96% chance of success.

If retiree B wants to spend ~6.7% for 29 years (the same spending plan at retiree A at that point) they have the same odds of success as retiree A. And given the availability of an extra year of data which wasn't know to retiree A when they started on their retirement, those odds are now less than 96% for either A or B.

If retiree B wants to spend ~6.7% for 30 years (a more ambitious plan than retiree A), then B's odds of success are presumably slightly lower than A's, although likely not by a measurable amount.

*Edit: based on the update, yes I agree with you, just plugging the numbers of retiree after one year of retirement have passed isn't going to provide an ideal estimate of success rate. This starts to get complex, so I'm going to start with four statements, I think we can all agree to (but let me know if you don't). *

#1: I'm not sure if we have a good way to quantify the *amount* of change in success rate based on the new information, but we know most failures of a 4% WR strategy do result from sequence of withdrawal risk early in the retirement, so a 1/3 decline increases the likelihood that we are in one of those failure scenarios relative to the odds of being in a failure scenario when retiree A pulled the trigger.

#2: Similarly, we know that if retiree A and retiree B spend the same inflation adjusted amount as a percentage of their starting portfolio for the same number of years, their odds of success are identical.

#3: We know that adding additional years to a FIRE scenario will never increase success rates, and sometimes decrease it. (So retiree B retiring for 30 years rather than 29 assumes some, unmeasured, additional quantity of risk).

#4: We know that spending a smaller proportion of your stash (especially at high withdrawal rates like 6-7%) decreases risk. (So if retiree B retires spending the same amount as retiree A -- and hence a slightly smaller percent of assets -- they reduce their risk by some, unmeasured, additional amount).

What we don't know is the size of any of these effects. If the size of effect #3 is greater than the size of effect #4, then retiree B has slightly more risk than retiree A. If #4 is greater than #3, retiree B has slightly less risk than retiree A.

But I think points #1 and #2 explain the parts of the original scenario that people were considering someone paradoxical, or representing a flaw in the logic of the 4% rule.