Food trucks have been around for some time and they will continue to be around. The street food culture just seems like it is experiencing a renaissance right now. I've heard, seen and tasted first hand so much variety and it's awesome! To be able to get a lumpia, bowl of pho, grilled cheese sandwich on the street, on the go and out of the blue is cool. But the level of popularity and the sheer number of food carts I believe will fade (probably not anytime soon though).
i think food trucks will always evolve and be around. i love taco trucks, and have yet to experience the fru fru truck, but i have nothing against giving those a try too.
....it's not a fad.