I think where
@Disneyhead'71 misses the mark is that he misunderstands the impact that new rides and attractions have at the Magic Kingdom. Increasing capacity increases supply, he's correct there. But at the Magic Kingdom specifically, it also disproportionately increases demand. In other words, if you build new attractions that can accomodate 30,000 guests, 35,000 people are going to show up to experience them. Those numbers are obviously 100% fabricated, but the idea that new MK attractions draw more people than they're able to absorb is a fact.
Further, all of this puts additional strain on Magic Kingdom's pinch points. Namely parade routes, fireworks, egress, and transportation. Even if you build tons and tons and tons of new stuff such that MK can handle 200K visitors a day, those people are all going to congregate around the hub for parades and fireworks, and a large chunk of them are going to try to exit the park immediately thereafter. There's a fundamental limit on the capacity of those pinch points.