... How many miles I rode up to this point is not ... relevent. ...
Sure it is. Billy might get 8000 miles out of a rear PR3, while Johnny might get 4000 out of the same tire on the same bike. Let's say Billy wore out the sipes at 6000, which means that he had 2000 miles left. If Johnny's sipes wore at 3000 (and he went another 1000), we can surmise that you have 25% life left (plus or minus). Theoretically, a more "sporting" rider gets fewer miles, but if the percentages line up, one could estimate for their riding style.
Besides, number of miles on a tire is VERY helpful to members doing searches for tire life prior to purchase.
Later, I'll write a post about someone heading west from Detroit at 60mph, and someone else heading east from Spokane at 75mph... and asking where they'll meet.