What is not “sustainable” is Saudi oil output and profits. They lost the fracking battle to the US, which has far more oil then they have and can sustain the rate for 50 years or more. Saudi has no choice but to take the money they have now and use it to transform their society.
Problem is, they can’t change their society quickly. They have an entitled “royal” class, plus their regular citizens are used to importing non-Saudis to do the actual work in the Kingdom.
Making Saudis actually willing to get their hands dirty will be a big job.