Most arguments about “locking the schedule” are really arguments about when, not whether.
Why it matters
A schedule that can be changed up to the last minute cannot be planned, because nothing in it is reliable. A schedule that is frozen a week out cannot absorb the ordinary surprises of the business — a callback, a cancellation, a customer who is suddenly away. Days out resolves that by making the answer depend on distance: far away, everything is negotiable; close in, only exceptions move, and they move deliberately.
How you notice it slipping
Tomorrow’s board is still being rebuilt at half past four today. Technicians check their route twice because it changed after they looked. Dispatch has no way to say no, because there is no agreed point at which the answer becomes no.
What to do
Pick a horizon per work type rather than one for the whole business. Recurring service can settle early because it is predictable. Reactive work needs to stay open. Then say out loud what may still change inside the horizon and who may approve it — a rule nobody can name is not a rule.