OK, thanks, but I still don't quite get the "using less resources on Day 5 than on Day 1" part.
If I ride less rides on Day 5 than Day 1, how does that save Disney any money? The rides still have to operate.
I guess you're saying that using fewer Fast Passes on Day 5 means fewer complaints from guests who aren't able to get Fast Passes?
At any rate, I guess time will tell if FP+ reduces guest complaints. (Seems like I've said that before.

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Depending on what you do, you physically cost Disney less than a Day 1 park guest on average.
Say you go to MK on your Day 1 - ride a bunch of rides, visit guest relations for a birthday button, stick around for the parade and fireworks, etc..
Well, every single moment cost them money to produce. Nothing is natural and sprung up from the earth - not the grass, not the trees, not the water. All of it cost money to put there. Plus staffing, etc..
So then Day 5.... You visit MK, ride a "few" attractions, don't stop by guest relations for a button, don't stop to see the parade, and don't watch the fireworks because you've done that already.
You've cost them less money. Yes they still had people at Guest Relations but only 3 instead of 4 to cover yours and all the other day 5 guest questions/button requests in a reasonable amount of time. Same for the parade and fireworks.. yes they still have it.. but they're not doing the parade twice like they do during busy seasons and they're not having to open the backstage gates and reroute people and hire more crowd control staff like they would if all the Day 5 guests were behaving like Day 1 guests.
And that's why you cant sell a ticket to someone else.. because it reverts to a Day 1 ticket when a new person uses it and thus the calculations for how much a guest "costs" is thrown all out of whack and costs Disney more than they expected to keep you safe, happy, etc..
Epcot needs 10,000 paying guests to stay in the black, btw. Otherwise it loses money just by being open.