You are correct... current Disney doesn't care if guests are at MK or one of their other parks. That is exactly why MK has received what you've listed.
That is the problem with the resort.
Walt Disney World used to be a world filled with possibilities.
Walt Disney World of today might as well be relabeled Magic Kingdom and Friends.
The place that use to care about the quality of Show they offered is now dictated by seeing how many spins on the MK turnstile they can get.
This model is what is allowing quality to slip further and further.
Honest question.
Do you think that the Disney of before did not believe the same thing?
The addition of new parks was meant to draw more people to the WDW resort, and make them stay longer, selling more tickets, merch. It was not some utopian dream to give people more with no regards to making money.
At the time, they had space to grow, the initial plans for the Florida Project were meant to grow to more than the one park. When they reached a point of deciding that the MK was reaching a large crowd level, they made the decision to add a new park and grow their attendance.
Going strictly on attendance numbers, attendance jumped by 10 million guests with the addition of Epcot, 5 million with the addition of MGM, and 3 million by adding DAK. Returns are diminishing as more parks are added. Adding something akin to a 5th gate now, would we even get the 3 million visitor bump that DAK added? I doubt it. Given the cost of construction of just a new land in DAK, how much would a full new park cost? How many visitors do you have to add to make that investment worth the money? I don't have those answers, but I'd bet that there are folks within TDWC that do.
They just know that the incremental bump, at this time, that they might receive in new visitors with the addition of a 5th gate would not justify the money spent to build that gate, plus the resorts required to house the new guests, plus the roads, etc.
The missions of the parks are the same as ever were. If they are holding themselves up to providing value to the customer as compared to the costs those customers are paying are up to the individual customers to determine for themselves.