It would be easier sure, but a film for this simulator is not that expensive to produce. The exterior theming and queue theming is where most of the money will be spent for whatever replaces ST.
Honestly Disney is moving at a perfectly normal pace for integrating new IP into the parks (Frozen had it's ride open right around 2.5 years after the films theatrical release). Going by your logic, why has it taken until 2018 for Universal to open a Fast & Furious attraction?
Just because an IP is new and hot doesn't mean that it will sustain itself over time. Hell, Mickey will be 91 by the time he finally has a Dark Ride.