"The primary reason we care about Saudi is oil."

This is true.

"US policy makers seem to view them as a necessary evil until we transition to renewable energy."

This is false. There is nothing at all necessary nor evil, relative to the United States. We use the middle east to establish a protection racket on oil flows to the rest of the world.

The United States gets zero, or near zero, oil from Saudi Arabia. The same is true for the UK which gets oil, almost exclusively, from the North Sea. We don't need Saudi Arabia for anything - certainly not as a "bridge to a renewable future".

We choose to hold the rest of the world (particularly Europe and Asia) at gunpoint with a "nice flow of cheap energy you have there ... sure would be a shame if anything happened to it ..."

"At some point after “peak oil” SA seems likely to experience extreme social upheaval ..."

Obligatory:

https://www.youtube.com/watch?v=hWeMpaC4VeA

The us imports roughly 9% of its petroleum from SA, (it's second largest single-source after Canada)[0], which is significant. However, I think if you polled policy makers they'd emphasize security interests over oil. SA serves as a powerful Sunni counter-balance to the largely Shiite Iran. They are huge purchasers of western arms, and often (explicitly) support western interventions.

[0] https://www.eia.gov/tools/faqs/faq.php?id=727&t=6

I had always heard that it was because it enforces the denomination of oil trade in USD, and so enshrines the USD as global reserve currency.

I agree, but also they provide a huge amount of corporate welfare in the form of arms sales and political welfare in the form of lobbying.

Yes, you're correct - I meant until the world transitions and oil is no longer such a strategic commodity to anyone, which could still be a hundred years away.

Oil is essentially a fungible commodity.

A reduction in Saudi production increases prices everywhere.

"The primary reason we care about Saudi is oil. "

There is also the Israel question which makes the US position pretty complex.