Thanks to the posters at WDWMagic, especially Tirian who offered a number of insightful considerations, I've continued to expand my opinions and ideas on 3D films, 3D environments and the role interactive experiences will take in the future of Disney parks.
For the sake of completeness, I will include my final post from the discussion board here:
In reply to Tirian, when you said that "films cannot literally drop you down a flume, walk you through haunted hallways, or give you whiplash", I totally agree - I only intend my argument to apply to tame, passive attractions like Pirates and Mansion. Simulating the tea cups won't do anyone any good, and rollercoasters aren't at threat because their appeal comes from the physical sensations, not what you see along the way, which is what dark rides rely on. With Pirates and Mansion, once you're seated you literally have no interaction with the world around you. In these situations, if the screen technology is advanced enough, how would you know the difference (obviously that's a big if). The portions of the ride that can be interacted with (say, the portrait gallery before you board Mansion) I don't expect to be made obsolete. The fact that you can reach out and touch this is, exactly as you said, what makes them vastly superior to screens. Its this reasoning that makes me feel tangibility needs to be massively expanded.
That said, I can imagine this happening before screens are advanced enough, if the other benefits screens can provide (computer generated imagery and so on) outweigh the perception that it's not real. Ideally, screens need to be photorealistic, simulate changes of viewing angles and be glassesless before I'd be entirely comfortable with them fully replacing sets. I don't know how likely this is.
Thinking about it, perhaps the Spiderman attraction at Universal's Islands of Adventure is kind of what I'm expecting to happen; screens disguised into physical sets - although (even with the massively commendable leap in realism), the screens can still be noticed as screens. When it's mainly the speed that avoids that realisation, slow moving rides like Pirates wouldn't stand a chance.
"As lazy as the idea sounds, Guests will at some point lose patience with the fact that their experiences completely depend on them."
Here, I very much agree with you. For decades now, Disneyland has been passive, television is passive, films are passive - and they're all doing fine. The interactive playgrounds I'm imagining would, for many people, by quite simply exhausting after a while. This is why I definately feel you're correct in saying "a balance of interactive, open-ended environments and thrilling, pre-selected experiences (e.g. Space Mountain) presents the best solution." And wow, won't it be amazing when it happens!
You mention the size of the showbuildings needing to be increased (and I find it funny you mentioned Florida, I'm a Disneylander myself so California is always at the forefront of my mind when I'm thinking about this!), and you're definately right. With more people exploring and interacting, the queues won't be people-eating and the parks going to need a lot more space. That said, if we compare the Pirates showbuilding to Snow Whites, there was no problem massively expanding the building size back in the sixties - I guess we'll just need to do it again, this time on a twenty-first century scale. Where this will leave the land-locked Disneyland is very puzzling.