Saud family have a lot in the bank, so they could hire someone else if USA politics got rid of their lobbyists who control much of Congress. Are they so weak that they would simply collapse without USA support? I would think they could just hire e.g. some ex-Pakistani military to keep the populace cowed.

Or do you mean to say that USA could easily replace the regime with a different, more-favored one? How did that work in Syria? We're withdrawing from Afghanistan now, and at this time Taliban control more territory than they held in 2001. USA military is a bit of a paper tiger, when it comes to achieving results via military action.