Keep in mind Disney's "contract" with those of us on the DP is for an entree, dessert and beverage on the plan. There is no obligation for them to allow for an appetizer in exchange for a dessert. You are always free to decline a feature of the plan, but Disney really doesn't have to offer substitutions as such.
I am a vegetarian (35yrs) and have done a lot of declining over the years. Sometimes we've gotten substitutions or a price break, but not always. I've taken the stance that it is my choice to eat this way and my responsibility to feed me. With that in mind I've either avoided certain places or events, lived with the options or simply eaten before or after an event.
If you really don't "value" dessert then do your DP "worth" calc assuming no dessert. If its that big a deal don't do the plan and pay for your meals OOP. That way you'll be sure to get appetizers when you want and won't feel like you have to eat desserts because you paid for them.