Saudi Arabia has a large military to deal with external threats. And they also have a large and separate National Guard to protect the royal family from coups and terrorists.

I... am stunned why people think this in any way supports the narrative of petrodollar-supremacy. Because neither oil nor dollars are implicated in that string of statements at all. Especially GP's summary of the statement--"we bombed Libya because they wanted to price oil in euros instead"--is at complete odds with the implied statement here--"Libya wanted to move from euro-backed currency."

Now it's important to note the context of the source. The diplomatic cables are--effectively--gossip reports. In general, they're not indications that anyone actually believes them, nor indications that their reports are causes for action. A report that a French president is concerned about threats to French suzerainty in its former African colonies is exactly the sort of thing that I'd go "right, I believe that... and I don't care." There is yet, to my knowledge, any evidence that this actually influenced US foreign policy in any manner whatsoever, let alone supports the petrodollar-supremacy narrative.

The amount of dollars kept in reserve to trade oil with certain petroleum exporters is a tiny proportion of total dollars in circulation

What is useful is having quadrillions of dollars of other payment obligations denominated in dollars, from the IRS to mortgage debt to IMF loans to international financial markets to people paying cash rent in Ecuador, and a central bank that can raise interest rates if it believes the demand for dollars is falling.

Sterling is still valuable, despite the fact it stopped being the main global reserve currency decades ago and the UK doesn't have any real ability to invade people to change that.