Here is a way to grasp the subject.
Imagine that with accumulated flight hours, the blades of (for example) the power turbine get asymmetrically eroded, resulting in a change in the effective "twist angle" of the blade and a reduction in its efficiency.
For this to result in, say, a 10% reduction in the power generated by the turbine disc would necessarily involve a significant thinning of the blades themselves, rendering them unserviceable and requiring their replacement.
This means that the blades would not get reworked to correct the twist angle at that point, because they would be beyond their thickness wear limits.