This totally cracked me up because I totally missed it, and it's really pretty obvious, isn't it?

Yep, if you don't get the DDP, then you still do have to spend
something on food, huh?
It's really simple. Setting everything else aside: whether or not it's a hassle to get ADR's, whether or not it's "too much food" and you never order dessert -- if you will spend at least $40/day on site on food per adult, not including alcohol but including tax and tips on TS meals -- then you will at least break even on the dining plan. And if you check your receipts, it's really not hard to spend $40/day.
11:30 am CS lunch at Tusker House -- $8 (no dessert)
soda with lunch -- $2
3:00 pm Dole Whip float -- $4
7:00 pm dinner at 'Ohana -- $26
tax for whole day -- $2.80
tip for 'Ohana -- $5.00
total for day -- $47.80
11:30 am CS lunch at Columbia Harbor House -- $7 (no dessert)
soda with lunch -- $2
2:00 pm Mickey Bar -- $2.50
7:00 pm dinner at Concourse Steak House:
salad (appetizer) -- $5.50
Prime Rib -- $21
Key Lime Pie -- $5.50
soda -- $2
tax for whole day -- $3.19
tip for Concourse -- $6.00
total for day -- $54.69
The dining plan costs $39/day per adult. These daily menus are not unusual, nor are they huge, ridiculous amounts of food. One CS meal, a snack, and one TS meal. I didn't even calculate dessert with lunch, and for dinner, I didn't pick the most expensive restaurants or the most extravagant menu items -- the Prime Rib at Concourse is $21, but there is a filet on the menu for $27. I didn't list any breakfast, but you could swap out the afternoon snack for a bagel or fresh fruit cup at the food court with a refillable mug of coffee for a light breakfast. Still a very reasonable amount of food for one adult, especially for one adult who is probably walking 10 miles a day!
If you are a DVC member and cook in your room, or if you brought your own car and plan on going offsite, then YMMV. But if you are staying onsite, took the ME from the airport so you aren't going anywhere else, and you can't/don't want to cook in your room -- the dining plan is actually a very good deal. Yes, you have to make ADR's for TS restaurants if you want to be sure not to waste those credits. But everyone has to make ADR's now if they want to eat in any TS restaurants at all, whether or not they are on the dining plan. It's just the way things are now, and not likely to change -- the restaurants are busy, full, booked solid, and Disney likes it that way.
Can you eat cheaper than $40/day? Yes. But you have to be willing to eat a lot more CS (nothing wrong with that if it's what you prefer, WDW has some great CS places), and it does require a bit of advance planning, too, if you want to keep your costs below that $40 mark.