This likely applies to upcharges at most casual food options:
I don't know the economics, but at least for dinner, isn't it more expensive to serve an MDR meal than any casual restaurant? So that being the case, I would try to encourage passengers to eat at Alfredo's. Planks/steamers might be different in that it's right in the buffet, and people could eat at Planks and still easily graze the buffet.
So maybe Princess COULD consider having Alfredo's free after 4:30 or the like? Profit wise, I'm not seeing a larger profit by discouraging people from eating at Alfredo's. Especially since it seems like staffing doesn't change there. Workers will be there no matter. This is different from the specialty restaurant options.