The rear tire finally wore out, after more than 12,000km of use. The centre was worn to a paper-thin sheaf of rubber, exposing the threads in several spots. Time to replace it with a new one.
Both tires are Specialized Armadillo Crossroads 700x38c. Armadillos have kevlar threads, making them extremely puncture resistant.
The tire on the left is a 2.5 year old, well worn. The one on the right is brand new.