I've thought about this for a while, and have a few observations.
1) The traditional MK parks are the greatest achievement in theme park design ever. Walt Disney cracked something when he came up with his formula. The industry has worked for 60 years trying to achieve similar success to the Magic Kingdoms, and currently only one park has.
Design AND marketing MAGIC. And it became ingrained in the psyches of generations of Americans. For my childhood, you could only visit Disney in the USA ... and later on Tokyo (but no one went there for Disney back in the 80s). It was a special product and every park was run to the absolute highest levels possible and the pricing, even after the Eisner/Wells increases began, was fair. It wasn't out of reach of so many.
So, it is easy to see why the castle parks are always the most popular. BUT ...
2) The MK domination is not a phenomena isolated to Orlando. DCA, WDS (this may not count...), and WDW Parks all have around half the attendance of their sister Castle Park. Even Tokyo Disney Sea, the park that has come closest to matching its neighbor has millions of guests between it and its MK. DCA which just had 1 billion+ dumped into it has half the attendance of Disneyland.
MK didn't dominate over EPCOT Center in the 80s like it has been dominating over Epcot in the 21st century. No, they were never equal. But the disparity wasn't like it is now.
TDS pulls in amazing numbers and numbers that EPCOT should be able to do easily. DCA was a disaster from the start, so it will likely take a good 5-6 years of more good buzz to start really putting up numbers (the new Marvel area may help, although Potter on the west coast will be open years before and SW will be going into the original too! So, DCA may never get as close as is possible. And let's not even talk about Paris's situation right now.
3) It's not always been like this, and it doesn't have to be like this either. Looking at attendance from the 1980s suggests there was greater equilibrium between Epcot and Magic Kingdom. Even into the 1990s Epcot continued to be closer to MK. Then things went sour. Tokyo Disney Sea proves that parks can be within the same league as MKs given enough time and money.
Exactly.
4) The Walt Disney World Resort is not a maximized asset by any stretch. Not even close. They have the potential to capture millions of more guest days with proper build out, appropriate management, and marketing. Enhancing existing parks are an incredible business opportunity. Unlike riskier foreign ventures they know the turf, new guests will benefit the sister parks, and it will help DS and the hotels. Fixing the other parks seems to be the clearer and clearer option everyday. Currently they're losing hundreds of millions of dollars from their inaction/slow action.
That is my point. I don't care how many Guests MK is getting. It is hugely successful. But how many Guests, including Guests staying at your own hotels are skipping one, two or three of your other parks? Or not checking out the water parks? There's a reason Disney has been planning a new water park since before RC was left to rot in the swamps and, yet, there are no plans to actually go forward with one ... or expand capacity at the two that they have.
5) How many more people can MK hold at its current capacity? For the last several years they've relied on consistent natural growth. When does it just max out? Can MK continue to represent a growth opportunity?
Depends. Open the dead zones. The food locations and retail venues that have been shuttered or (look at Adventureland) moved out into walkways and courtyards like a flea market. Add live shows and entertainment, so people aren't always running from attraction to attraction. There's room for more.
Of course, there is also PLENTY of expansion room and, yes, this park needs another two people eating E-Tickets. No, not as much as the other parks just need something. But the need still is there to spread out the Guests. Another E-Ticket in Adventureland or Tomorrowland might pull some people from crowded walkways.