Okay, I was going to write this up later. But with all due respect, I experienced seven anxiety attacks while reading your post. So here's what I say. And I'd love to hear if and how you/others disagree.
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Fifty years ago, the idea of spending a week at theme/amusement parks was completely foreign. "Why would anyone want to do that... wouldn't that be too much of the same thing?" they'd say. Disneyland was built with the intention of being a day-trip (or one night) place, just like any amusement park or carnival. Magic Kingdom was also built with the intention of being one park in a resort of other things (like a city of tomorrow).
When Disney decided to add more parks to WDW, they had to sell people on the idea of a theme park vacation. So they made each park fundamentally different.
It's fine to have one Magic-Kingdom-like park, but two? Three? Four? Think about how terrible UO would be with Universal Studios Florida, Islands of Adventure, Peninsulas of More Adventure, and Plateaus of Epic Adventure. I already can't tell the difference between the two parks... four of them would feel rediculous, confusing, and probably unappealing to the general public.
Today, the theme park vacation is so ingrained in our culture that things won't change overnight. But if guests begin to feel that it's too much of the same thing, they could begin to spend less time or fewer vacations in Orlando. Or at DL. Or at any place that wants you to go to more than one park. As someone who has no interest in seeing the downfall of the theme park resort, I really think each park needs to be fundamentally different.