You could have asked for the operations manager or front office manager, complained and, if necessary, read the applicable provision of the terms and conditions, which differentiate between "continental breakfast" at non-resort properties without a lounge or without an open lounge and resort-designated properties, the latter of which are required to provide a "breakfast in restaurant" if chosen at check-in.

You could have invoked the guarantee. If they still didn't budge then you could have filed a complaint with Marriott so at least the record is preserved. Unfortunately, I suspect most of the guests experiencing noncompliant breakfasts aren't registering a complaint with Marriott. I would also make it known on Trip Advisor that the property is playing games on the breakfast.

By the way, did you get an alternative amenity from this property charging a resort fee that included internet access?