I can see that this would make a big difference, and I do wonder how many visitors these days are there because they love Disney more than the parks in and of themselves. When I first visited Disneyland and all this began, Beauty & the Beast was just coming out in theatres which is suggestive of the fact that Disney was on the cusp of becoming the cultural behemoth that it soon became. Indeed, I vividly remember wondering before we visited what they could possibly have at Disneyland as I didn't think Disney had really made many big films. What they did have blew my little 10 year-old mind! Perhaps in some way the parks had to be able to stand on their own creatively as a Fox & the Hound Land wasn't going to bring the hoards running? Nowadays the dynamic may well be the reverse: crowds show up wanting to see their favourite film and are disappointed if it's not represented.
An excellent analysis. Thankfully, the non-IP approach is enshrined in enough of the headliners and lands that I don't think that aspect of the parks is really at risk of being lost. I can't see how Main Street USA, for example, could be IP'd into something else, and even World Showcase will remain fundamentally true to its original theming and purpose.
Who knows. Either way, I'll remain bitter and twisted!