Isn't it. Do tell me how Iran can invade KSA. Iran doesn't have any air force or navy to talk of(which are very critical in taking the war into an adversary's land). How on earth can Iran attack KSA? At best Iran will launch missiles at the Saudis, but that's just about it to be honest.
I think the most realistic thing that can happen will be for both sides to be engaged indirectly through proxies like they have been doing for sometime now, they can merely increase their support for their proxies and intervene more forceful against each other proxies(i.e Iran in Syria and Iraq, and Saudi Arabia in Yemen ).That's about it. Both sides know a direct confrontation or war with each other will be catastrophic for both of them with no clear winner like Iran-Iraq war.
So better to fight till the last proxy.