A subclass that no longer differs meaningfully from its parent; the agent navigating the hierarchy traverses indirection for no behavioral variation.
The subclass folds into the parent; the agent reads one class instead of a degenerate two-class hierarchy.
Before the refactoring
class Employee {}class FullTimeEmployee extends Employee {}
After the refactoring
class Employee {}
The agent's reasoning pays the hierarchy ceremony cost on every reference; future maintenance touches both classes for any change.
If the subclass documents a future variation (extension point, planned divergence), collapsing destroys it; the agent that collapses without checking forecloses options.
Smaller hierarchy; less ceremony; the agent loads one class instead of navigating a degenerate two-class chain.
Collapsing subclasses that document planned future variation destroys extension points the agent will need to re-introduce later at higher cost.