MBS recognizes that the situation with the “Saudi royals” is unstable. The “royals” have an especially high birth rate, with each new generation having the expectation that they will have the same opulent standard of living as their parents.
It is not sustainable, and there would be “internal unrest” at whatever point the spoiled royals got their income cut. Might as well do it now.
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.