Originally Posted by FBinNY

With spoon brakes, the steel spoon wears faster than the tire. The tire picks up abrasive media (grit) from the road and uses that to destroy the spoon. Also consider the difference in working area, 3 inches of spoon vs. 80" of tire.

Anyway most tires do have wear indicators in the form of a clear or tan gum layer between the tread rubber and tire cords. Even when they don't, the lighter cords show up in good conrast long before they're actually compromised.