“I see little possibility of a ground war. It would be all missile, all air.”
They both have significant Naval forces, and huge naval interests - both of them ship the bulk of their oil out by Sea, through the Arabian/Persian Gulf and Straits of Hormuz.
Iran has it’s biggest Naval Base at Bandar Abbas, right in the Straits of Hormuz. It could well be the main focus of a direct war between the Saudis and Iranians.
Oh don’t get me wrong, if they started up a war it could get quite brutal and thoroughly destructive, it would be triply existential for both sides: oil + country + religion.
Though oil prices would skyrocket, the world would not be worse off if both those countries were converted to smoking holes in the ground. Though the Iranian people by and large are not our enemies.