Interfaces are only necessary to properly abstract away from implementation details that have different change driver assignments. Else it's overkill and arbitrary. This doesn't invalidate hexagonal architecture, it just provides the actual good guideline to know if abstractions and information hiding are necessary.

Check the Independent Variation Principle paper for more info: https://doi.org/10.5281/zenodo.17677316

The IVP provides two directives that help evaluating objectively design options, based on actual business decisional authority structure, not some guy's intuition. With the insights of the IVP, you'll be able to decide effectively.

The paper is long, but you can skip to the parts that you find interesting