Actually,the inside of the wiper arm is very soft metal, so when the arm wears out, the metal will have collected in the grooves, making the grooves look like they have worn. The techs that "replaced" mine cleaned the grooves with a sharp hard metal tool. Then they attached the new arm and it worked fine.
I had thought it meant a new motor or shaft; wasn't needed.
Dave and Beth
2015 Cornerstone 45J
2020 Ford F-150 Lariat