The gigantic arms deal with SA is a phobic reaction by the Saudi leadership to Iran's incessant threats and millenarian messianic ambitions to occupy Makka and Madina, destroy the Kaaba, and unearth the grave of the Prophet.
Moreover, Saudi Arabia has apparently come to the conclusion that it can only rely on itself and that erstwhile friends such as Sissi's Egypt are, to say the very least, unreliable as strategic allies against Iran. The Saudi leadership seems to have realized as well that Sissi's heart was effectively with Iran not Saudi Arabia. The Saudis have also realized that Egypt only views SA as a milking cow pumping dollars to prop up Egypt's hopelessly collapsing economy. This is my interpretation of the huge arms deal.