You're not wrong about the derivatives, but what you are wrong about is that that's what will sustain the Sauds. Most petroleum products are the kind that get burned as fuel and that's where Saudi money comes in. Even if the derivatives are still used, they're not the major use of petroleum. Furthermore, the problem isn't about energy, it's about their economy. Even their "diversified" domestic industries are all subsidized by contracts paid for with oil money. The Binladin Group has billions in revenue, yet those billions come almost entirely from oil money. Saudi Arabia is paying itself by taking out of its left pocket and putting in its right one.
The Saudi Economy will not have a great time when oil drops. Almost all of its industries are tied to funding brought in by the oil. The Saud dynasty however will hoard its cash in offshore bank accounts and when things go to hell, they'll hop on their yatchs or jets and go live in luxury in some Western country.