Agreed, but a cambelt is not safety critical, more wallet critical.
There is also a difference between systematic failures (wearing out etc that is predictible, repeatable and can be tested to get a median) and random failures. With safety critical lifing you are in a statistical game and will apply a safety factor to replace the item before it systematically fails. Random failures are extremely difficult to predict, which is why the system is not foolproof.
regards
Retard