Mohammad bin Salman must retain the support of the heads of the army, secret police, and such or he will be replaced.
The Dictator's Handbook[0] discusses this topic in detail.
The Rules for Rulers[1] is an 18 minute video that summarizes some of the principles of the book.
[0] https://en.wikipedia.org/wiki/The_Dictator%27s_Handbook
[1] https://www.youtube.com/watch?v=rStL7niR7gs