Ok, so guests will be attracted to a great experience and themed area, not just the IP
The key thing that made the Harry Potter lands so successful is that it brought to life the lands and characters people had watched on film for many years. Had Universal done a Six Flags and built a roller coaster called "Harry Potter's Patronus of Truth" or whatever, I don't think it would have been nearly as successful, because the Harry Potter IP is completely inconsequential to that ride experience; it's purely a name to give the latest, greatest thrill ride. But what Universal did was take the universe from the Harry Potter films and books and bring it to life and allow people to be immersed in that world with those characters. Half of what made the Harry Potter lands so successful was that those worlds had already been introduced to people and ingrained in their conscious, and people had dreamed of seeing those worlds come to life for years and years. If Universal had built "Home-Grown Wizard Boy Land" instead of "The Wizarding World of Harry Potter", it would not have been nearly as successful.
You cite Six Flags' use of IP, but the key difference is that Six Flags is not known for its IP use or the fact that it has Looney Tunes and DC. Six Flags are known for big, awesome coasters and thrill rides and the quintessential American amusement park experience; Looney Tunes and DC are a side accessory. Disney and Universal, on the other hand, are known for their IP use, and the parks are becoming increasingly associated with the IPs of the respective brand. People go to a Disney park and expect Disney IPs. People go to a Universal park and expect Universal IPs, Harry Potter, Nintendo and the like.
The thing that Universal and Disney are primarily known for these days is immersing people in the worlds from their favourite TV shows and films, and letting people be immersed in the universes of their favourite characters. In this day and age, I think people would be disappointed if they went to Disney World and couldn't be immersed in the universes of Star Wars, Marvel, Cars, Disney princesses etc. That may not have once been the case, but people have increasingly grown to associate Disney parks with Disney brands, whether we like it or not.
The difference there, though, is that those Parisian streets were part of the wider package of Epcot when it first opened and were never designed to be a draw on their own. If those Parisian streets had been built as a standalone land, they would not have had nearly the same appeal as an equivalent IP land.
You also have to consider all of disneys parks are a theme in them selves, animal kingdom is about protecting wildlife and conservation, epcot is was about connecting people bringing ideas and inspiring people, hollywood studios was about movie making, none of these IP integrations work with that and it can be very jaring to go from toy story six flags with fiber glass models (I have been here before you say it) then imediatley into galaxies edge. This can be seen with guardians of the galaxy in california they used to think about how the ride would look through out the park, but no guardians needs to go here, thus lets paint this building to not fit in and be seen from a large amount of the park breaking a lot of immersion.
The thing with that, though, is that some of the original visions of the Disney parks grew very outdated very quickly.
With Hollywood Studios, for example, the whole premise of the park was to "bring the magic of movie making to life"... a premise that was quickly made redundant by the emergence of DVDs with behind the scenes features and has been made even more redundant by the rise of social media, YouTube and the like. People can see how movies are made by simply watching a DVD, or nowadays by a simple tap of their finger to log onto YouTube or social media. Why would they want to come to Florida to see movie making when they can see it from the comfort of their own home?
I'd also argue that Epcot's original premise has grown somewhat outdated. Future World faces the age-old "Tomorrowland problem" in that tomorrow always comes, and any notion of "the future" quickly becomes outdated. World Showcase has perhaps suffered less, but I still think even the original vision of that is arguably outdated to a degree. It was designed in an era where international travel to faraway lands was far less accessible, so the educational "advert for the country" pavilions format worked. But in an era where international travel is far more accessible, do people really want to go and see a caricatured advert for countries at Epcot when they can easily go and see the real thing? I'd also argue that increasing political correctness has dated elements of World Showcase; as much as it's a lovely area, bits of it haven't been updated in years, and it shows...
Consider this; for quite a few of their parks, Disney has recently had to pump exorbitant amounts of money into them to completely reinvent their identities due to the original premise becoming dated or outstaying its welcome. It happened to California Adventure. It's currently happening with Walt Disney Studios in Paris, whose name has even changed to reflect a totally changed identity. It happened to Hollywood Studios. It's also been happening to Epcot to an extent too.
