In our experience, you cannot sub a dessert for an app. It’s not allowed. In the past the DDP included tip, app, non alcoholic drink, meal, dessert, unless it is breakfast. In the past few years they removed gratuity and the app. IMO, it’s unnecessary to have the app. You are way too full to do app, meal, dessert depending on the meal picked. I know many prefer this option, but (according to fil, mil, and dh who have all had the old ddp in the past) it’s too much food and you end up making choices on what will stay good while you walk around the park, or what will fit in your backpack. They hated being limited, though they would liked to have seen gratuity stay in place.
I think, Disney is trying to save money (and make it too!) by eliminating the gratuity and app portions. They can hike the price, yet you still feel (and in some cases are) getting a good deal. Not everyone knows about the ‘old’ ddp and therefore will always think they are getting a deal.
I myself, just got back from WDW about 6 hours ago, and used my CS ddp to it’s fullest extent possible. There were a few times the meals added up to well over $40 at a single sitting. Snacks were just the icing on the cake.
For my family we like the meal plan, it eliminates the need for extra cash, etc. Plus we always averaged about a $10-$40 savings per day depending on the meal plan picked.