The preplanning spreadsheet you've done, I'd argue, is an illusion. People base their costs on what the DDP provides, not what they'd actually eat. Would your entire family be eating desserts at QS restaurants?
I agree most do, but I don't. I base them on what I'd actually eat. I factor in things like we'd only buy 2 sodas and 2 desserts and share them, not 5 of each. My estimate of a QS point is $13. I think that is pretty reasonable. That's enough for a typical $10 meal and $3 soda -- I don't plan on always eating at Flametree or buying QS desserts. I also only factor in the refillable mugs that we
would buy (2) not how many we'd get (5).
Are you not changing your dining behavior to fit the plan?
Not at all. On this particular trip, we happen to have 7 points worth of buffets planned. Not because of the DDP... we picked our restaurants long ago (and added the SW when they came avail) then decided to do the DDP because we had so many character meals it made it worth it.
Are you opting for the steak just because it's the most expensive thing on the menu?
At TS places, I would typically order the steak, seafood, or lamb... as would my wife... but in this case our points are across the board buffets so this is not really a consideration.
Are you going to be one of the people rushing around the gift shop on your last morning trying to use up your snack credits?
No, we did that on our first trip cuz we were stingy w our Snack credits all along and paid OOP for anything not maximizing the snack value (mistake). On our 2nd trip, we learned to use them often along the way, which included tons of $4 smoothies. Thus I now peg snack credits at $4 not $5.
I'd be interested to see your spreadsheet.
Sure! Be happy to have the critique. If you can show me that it is overpaying to use the DDP I will be floored. I'll PM you.

I do agree the DDP is more about prepaying and disconnecting from the expense later... and is usually not a savings, but for the right mix of meals (most particularly - character buffets) it can be.
TIW is also great (and I would consider it if I was eligible, which I'm not, which sucks)... as long as you don't need the blacked out restaurants or blacked out dates.