Stupid question huh?
Saudi Arabia has a population of 32 million, 40% of which consists of foreign non-citizens who may leave at any time. Suggest you check out some expat forums to see how unenthused most of these people are to be making a living in the Kingdom. Even those expat airline pilots who are bribed with very high rates of compensation seem lukewarm about living there.
The population of citizens is around 19 million. Nearly half of these people would need to live in this monstrous project at its planned build out capacity.
I also doubt that desalination is a viable solution for a city the size of this one for more than a brief period.
White elephant anyone?
Waste of the Kingdom's resources?