They're going to find the results of all that money very underwhelming unless they secularize the legal system.
I am pretty sure that they will be able to handle the Wahhabis inside their country...they can buy off most of the "scholars" and the behead the rest. But not sure that they can keep the masses happy (revolution?) or that the city will function as planned. $500 Billion is a lot of money and there's plenty of competition. But then, they are desperate.
Maybe start talking about petroleum era going away when say, 10% of vehicles in US are running on electricity?
Whether they will be successful at creating a semi-utopian city is another story. I expect it to have the same warts as the rest of the region in regards to migrant labor, human rights and civil rights.
How different is that though? We are bombarded with news about electric cars all the time, but the reality is that we are still a very long way from the world (Remember, US is only 4% of the world's population) embracing electric vehicles.
So while eventually the world is going to move away from petrochemicals, it is too early to say we are anywhere near the end of petroleum era, even from the financial perspective of Saudi Arabia depending on petroleum. Saudi Aramco is a behemoth whose value and cash flow is insane even keeping in mind how the country hemorrhages money by unnecessary expenditure. It's market capitalization is disputed in the range of trillions.