Before the premonition kicks off, multiple events have already occurred to contribute towards the roller coaster derailing. Frankie's camera is the secondary cause of the accident. The ride attendant forces down Lewis' harness which sparks a link in the hydraulic wiring below, this being the primary cause of the accident. Throughout the entirety of the Final Destination franchise, Death is known to prolifically act with both a Plan A, a Plan B, Plan C etc. We know this because the very fact that it puts into action a new design after a visionary foresees it (the new design being the list the survivors are on throughout the film) informs us that it is prepared to put alternate plans into action. In this case, Death would have upon Wendy and her group leaving spawned a new design, one which wouldn't require the presence of Frankie's camera and the main trio.
But remember, the hydraulics began leaking before the ride began and before Wendy awoke from her premonition. As she freaks out, the ride remains stationary for a prolonged amount of time, much longer then originally foreseen in her premonition, meaning that the wiring has been leaking 10 times more fluids then originally proposed. Thus, the secondary cause of the accident (Frankie's camera) was not required, due to it only speeding-up the process of the roller coaster derailing. Ironically, this means that the roller coaster could have crashed way before it originally did in the premonition (after the loop). If you look carefully after Wendy and Kevin look at the roller coaster derailing, it is collapsing at a sharp turn about 200 feet (61 meters) or so upwards. This part of the ride occurs right after the first drop, which is only about 30 secs or so after the climb. Overall, the roller coaster was part of Death's Design; regardless of who foresaw his plan and got off the ride, it still would have derailed.