In 30 years of repairing these units I have never found one to have brushes and commutators that were worn beyond the service limit. The failure in these units is the solenoid contacts. These are not shown in the supplied picture. These contacts take a beating; they generally last about 100k miles. Almost all these starters can be repaired by replacement of the two contacts and the plunger. Our standard repair is to replace just these parts, clean it well, lubricated as indicated by the procedure and then return it to service. We have done hundreds with this method and have not had one return for other problems. We have replaced contacts twice on a few.
The newer Denso starter designs are quite different and need a more comprehensive repair.