That's Engine Condition Monitoring
FA. A lot of parts such as compressor and even some turbine blades can be reworked using plasma spray techniques, nevertheless, many parts - turbine and compressor parts in particular - still have finite lives measured in operating cycles and are scrapped when their life limit is reached.
And yes, we do often stagger engines so that they fall due for work at alternate 'C' Check inputs. Not every operator can afford to have loads of spare engines lying around waiting for a pylon to hang onto. It makes managing the lease return conditions quite a complicated task though...
