It's kind of funny if you think about it. The arguments used to point out that the sensible diet isn't a value on the dining plan and therefore Disney is pushing the non-sensible diet can also be used to support Disney if you take the Dining plan out of the picture.
Since the sensible options are cheaper (thus making them less of a value on a dining plan) Disney is supporting the sensible diet by charging less for said options. A sensible day may cost $40, while a not-so-sensible day will reach the $50-55 range. Stretch this over a typical 5 day stay, and you can save upwards of $100 just by eating more sensibly. On this point, it surely sounds like Disney is supporting sensibility.
Of course, they are hard selling the dining plan as of late, which goes against the sensibility argument, but it still stands that the sensible options (man, I need a thesaurus.. and a spell checker...) are cheaper than the more outlandish options.
Best of both worlds for Disney? Probably. They can cash in on people who aren't making ideal choices and claim that since their healthier options are cheaper overall, they are supporting a more sensible diet.
As to making yet another Dining Plan? The CMs on the phones barely have a handle on the 3 primary ones offered now (if at all!). The guests, likewise, have very little handle on the plans when they are first presented with them. And lastly, since there aren't any specific policies in place, most of the times the restaurants are free to do their own thing with regards to what is included and what is not (Soups/Salads, Milkshakes, Substitutions, etc). A fourth would only compound this issue.
Lastly, how would it even work? You could cut out Desserts, but that wouldn't make entree selection any better (where the better value is still the larger meals). In any pre-paid set-price system, the more expensive meals will always be the better value. Since the healthier options are typically cheaper than the no-so-healthy options, there's always going to be a deficit. We could make the healthier options more expensive to change their perceived value on a plan, but that wouldn't make much sense either. Alternatively, you can drop the plan's price enough so that the sensible options are a bit of a value, but to do that, you're going to have to drop the price by more than the cost of a dessert, making this plan not scaled the same as the current three. (And thus, pushing the
DDP off to the side as this becomes a better deal even with OOP desserts). The last option would be to make it cheaper, but limit menu selection, which may work, but again lead to much confusion and issues with the restaurants not being consistent.
As for the original topic, it's been stated that desserts are cheaper to produce, easier to "mass produce", and removing a whole course (rather than giving the option) improves table turnover (opening up more slots in the day for more individual groups). There's also something to be said that by removing the first course, more people would likely pay OOP for it vs Dessert.