In my opinion necessity of Input Boundary abstraction depends on Stability metric of component, mentioned earlier in this book.
Stability metric explanation from the book
• Fan-in: Incoming dependencies. This metric identifies the number of classes outside this component that depend on classes within the component.
• Fan-out: Outgoing depenencies. This metric identifies the number of classes inside this component that depend on classes outside the component.
• I: Instability: I = Fan-out / (Fan-in + Fan-out). This metric has the range [0, 1]. I = 0 indicates a maximally stable component. I = 1 indicates a maximally unstable component.
Also definition of The Stable Abstractions Principle(SAP) will be useful in context of the question:
A component should be as abstract as it is stable.
Answer to the question using following explanations:
The harder component to change
=> The more dependent classes from different components it has (the more it's stable)
=> the more abstract it should be
=> the more Input Boundary abstraction necessity