I think when you rely on one IP to carry an entire park that's where things have a higher risk of going wrong. As Teebin said, MK does just fine without one obvious main draw. Sure there are a lot of draws, but the park doesn't overly rely on any one of them to get by.
Perfect example of how this can be unhealthy for a park is DHS. Once SW:GE opens next year, the park is gonna be packed on opening day with people waiting to get in. But what happens if both rides in SW:GE break down at the same time? it basically breaks the park.
On the other end of the spectrum, Disneyland will be packed, but the park isn't relying on it like DHS is. This is simply going to be a fantastic addition to an already well fleshed out park.