Well, truthfully, the relevancy of the source material shouldn't hold anyone back from creating a theme park attraction. What really matters is if it can be a great concept for an attraction. Great attractions have long lasting appeal even if their source material isn't well known anymore. A few examples: Tower of Terror, Splash Mountain, Popeye, and Waterworld.
Probably the best example would be Mr. Toad's Wild Ride. Back in the 50's, Mr. Toad was considered irrelevant even then. But that didn't stop Walt. He knew that the idea of riding around recklessly, like Mr. Toad himself, is brillant for a theme park attraction. And to this day, Mr. Toad's Wild Ride is seen as a Disneyland classic.
So yes, if an attraction is executed brilliantly and the good word of the mouth gets out, the people will come, no matter what.