The Saudi-Iran rivalry is less an ancient religious conflict and more a modern geopolitical proxy war cloaked in ethnic (Arab vs. Persian) and sectarian (Sunni vs. Shia) garb. The two countries are on opposing sides of horrific conflicts—in Syria, Yemen, and Iraq—that have caused over a million civilian casualties, the greatest refugee crisis since WWII, and the proliferation of Sunni Jihadist groups such as ISIS and Shiite militias to counter them.
The most powerful man in each country—78-year-old Iranian Supreme Leader Ayatollah Ali Khamenei and 31-year-Saudi Crown Prince Mohammed bin Salman (MBS)—are a study in contrasts. Khamenei is a deeply traditional cleric who cautiously rules a predominantly modern society; MBS has a modern outlook and bold ambitions but presides over a deeply traditional society.
The bottom line: Khamenei's death has been anticipated for over a decade, but it's equally plausible he and MBS will spend the next decade vying for preeminence in the Middle East. The cynicism of an autocrat who has ruled for 28-years, coupled with the assertiveness of a young leader eager to prove himself, means their rivalry is more likely to escalate rather than deescalate.