I think another aspect of complexity is that your customers, either internal or external, have a very specific idea of what they want, even if their idea is trash. So your product needs to be flexible(complex) enough to support all the use cases conceivable. Going to a customer and saying "trust us, do it this way" will make them lose interest fast.
I work on a small internal ERP, and our new UX guy said “nobody needs all this info”. And I said “agreed, nobody uses more than six columns. But the key stakeholders can only agree on 5.”
I think that’s the key: nobody needs 20 buttons on a microwave, but some people love defrost or popcorn or whatever.
