The main issue is the time it takes to load.
Let's say 15,000 in a hour to match peak loading, that is 250 per minute. Currently we are looking at about 2 minutes to load a train, such as a public trolley or metro rail system. So you would need at least multiple load and unload platforms.
Currently, a peak hour can go over 40,000 if Disney closes both parks at the same time. Now Disney tries to stagger ending times of main events, such as Fireworks and World of Color, to lower that number. Currently, you have 2 exit routes, to the East (Toy Story Shuttles, the ART routes, other shuttles, taxis and rideshare, plus walking) and West (M&F Trams, plus walking through DtD.
I agree that loading is the bottleneck and always will be. More people can fit on longer trains, but it also takes people longer to walk to the far end of a longer platform. Considering the passenger volumes, trains would dispatch in less time than it would take to wait for the next train, so there would be no incentive to walk to the far end
This is where a system like a gondola or PeopleMover with larger cars would excel, dispatching groups of 30-40 people every few seconds. This would also reduce the surges of arriving guests at the other end, and would help to smooth out guest flow considerably. If a gondola doesn't enough capacity on its own, they're cheap enough (less than 1/4 the cost of other fixed-guideway systems) that 2 parallel systems could easily be built, allowing off-peak closures of one for cost savings and maintenance
Also, the costs are high, and the city won't help in running it, you might get some small public right of way rights to allow bridge/track supports, but that is it. Disney would have to build it, then cover the costs of operations. And what does Disney get? They more than likely wouldn't charge to ride it, so it is clearly a money losing option, and for what gain?
Sorry, I simply don't buy this excuse. They're able to afford running a transportation system to their existing parking structure and another system to their existing far-flung parking lot, so why is it any different for this new garage? Heck, in Florida, they manage just fine with the world's busiest theme park that's more than a mile from its parking lot and has a large lake in the way.
Disney is charging $20 per car to park, and well over $100 per day to visit their theme parks. They can afford to run a transportation system connecting the two just fine. Yes, it's more expensive than forcing everybody to walk, but that doesn't mean that it's an undue burden on Disney. If Disney thinks that getting me from their parking lot to their theme park to buy their food and merchandise is too expensive, I have no problem taking my vacation dollars elsewhere
The 12,000 comes from the design of the original Eastern Gateway bridge, which takes into account let alone Pumbaa, but also all the folks that would use the Transportation Hub, plus those who walk from their Hotels.
The city would require a new project to minimize the impact on public sidewalks and roads, so you can't just say they would walk a few blocks. The city would require multiple pedestrian bridges to be built for approval of the project. And once again, Disney internally would -ask, what benefit do we get for building these bridges, which would be public right a ways to access non-Disney businesses.
The city would require multiple pedestrian bridges for a transportation system that dumps people in the Eastern Gateway, but wouldn't require any for a pedestrian route that dumps them there? Regardless of how people arrive at that point, I would think that they would need to be distributed throughout the surrounding area in a similar manner. Regardless of the mode people take from the parks to the Gateway, the Gateway isn't their final destination; similar solutions should be required for distributing guests either way
As for the M&F trams, if the system runs at maximum capacity of 4 load/unload platforms, they can get a tram off about one a minute. Of course, Disney runs as low as possible, as they much prefer to have folks walk through DtD.
Do they plan to do this on a regular basis in the future? I don't think I've ever seen more than 3 platforms in use at a time, even during peak-of-the-peak periods. Even so, that would only get the tram capacity up to around that coveted 12,000 mark, which using Disney's math seems inadequate for a 16,000+ space garage
And I've never seen many people walk through DTD, simply because the path isn't terribly clear. It was easier back when the side path behind Wetzel's Pretzels was viable, but the new security measures force everybody to take the longer and less clear route. If there is any conscious effort to drive pedestrian traffic through there, why is there no wayfinding signage to direct people? And why isn't the northern half of the route through the parking lots more pedestrian-friendly? Sorry, but I don't buy it. There may be any number of reasons that they don't run more trams, but encouraging DTD foot traffic isn't a major factor