A packed park won't necessarily lower overall revenue, it WILL lower per cap spending, which is also a measure of guest satisfaction. When people are happy, they spend more money. When they have to wait in long lines for everything, including food, drinks and souvenirs, they are far more likely to skip it and decide that it's not worth waiting in the line to spend the money. Per cap spending absolutely drops when the parks are crowded- sometimes dramatically.
Which is why a park can often make just as much money, sometimes even a bit more, if they have a small drop off in attendance compared to a crowded day. Even with photopass at the time we sold more photos in the parks, we would sell a lot more on moderately busy days than we would on packed days because people were so sick of waiting in lines that they didn't want to do it anymore. At the park my wife worked at in merch, her sales were down on crowded busy days compared to moderate days for the same reasons. She would make more money at her store on a day that was at 65-70% of capacity than she would when it was 85% of capacity.