rigidity (in-flexible, un-adaptable) is due to the fact that a single change to heavily interdependent software begins a cascade of changes in dependent modules. When the extent of that cascade of change cannot be predicted by the designers or maintainers the impact of the change cannot be estimated. This makes the cost of the change impossible to estimate. Managers, faced with such unpredictability, become reluctant to authorize changes.
Fragility (non-robust) is the tendency of a program to break in many places when a single change is made. Often the new problems are in areas that have no conceptual relationship with the area that was changed. Simple changes to one part of the application lead to failures in other parts that appear to be completely unrelated.
I think this “change-impact” analysis would help answer the question “how to reduce subclass/superclass dependency”