For better or for worse, I think themed entertainment has just evolved in the past 60 years. We used to have lands that followed a specific theme which featured similarly themed experiences living together. But thanks to advances in show design and technology, guests can literally feel as if they were transported to a completely different world (i.e. Harry Potter, Carsland) where everything ties back to a sole creative universe. While I think these types of endeavors are more of a gamble and risk, I think when done right, it allows for a far more immersive experience.
The biggest problem with these types of singular IP lands, IMO, is -- what happens if/when guests eventually lose interest in the IP it's based on? With a single attraction in a non-specific IP land, you simply shut it down and put something new in it's place -- or at the very least, re-skin it to something more relevant. What happens when guests grow tired of an entire land??
In the case of Cars and Avatar, I don't think either franchise has proven itself to have that kind of longevity. Star Wars (and probably Harry Potter for UNI) I think certainly has. It's been huge since 77 and wildly successful at Disney parks since the mid 80s. It is a modern day fairy tale up there with Fantasyland's greatest hits.
We've entered a new era of themed entertainment. One where themed lands and IP lands are going to have to co-exist and be enjoyed for what they are.