OP here. I probably shouldn't have made my argument sound quite so mathematically rigorous. :) You're right that a well-designed UI can decrease the complexity

OP here. I probably shouldn't have made my argument sound quite so mathematically rigorous. :) You're right that a well-designed UI can decrease the complexity of additional features. That said, I think it's usually true that additional features have a larger impact on the complexity of a UI than the complexity of an API. Not as a universal rule, but as a general guideline.