"Given the costs involved, this should be designed and operated to last the lifetime of the station, despite the harsh conditions."
Maybe, but most of the engineered products you're familiar with have been through many generations of design, operation and improvement, as well as competing against similar devices; so they are the result of accumulated wisdom on solving a particular problem.
The arm on the ISS is a second generation product (the first was on the Space Shuttle) and the opportunities for inspection and maintenance are pretty limited.
Even if a product is perfect and built to last a lifetime, it can still require replacement if it gets enough abuse from the users...
Biting the hand that feeds IT
