That also means that the existing "culture" for theme parks is pretty much zero, and it remains to be seen how attractive it will be in the long run. The Dubai - Abu Dhabi area also has about 8 million people (and a buttload of tourists) and a park featuring the world's fastest rollercoaster, and its parks still aren't doing particularly well for it.The fact that this might actually happen is nuts. Though, maybe not, considering that Riyadh has about 8 million people and there is no competition outside of the UAE which is a 2 hour flight away.
That's why I'm kinda baffled why they choose to build this. Theme parks haven't been a smashing success in the UAE, even those that feature record-breaking rollercoasters, and the desert of Riyadh isn't really conducive to outdoor activities. It will take quite a shift of the population's attitudes to make a lot of money from this.