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They have been reality for many years. A compass built into the car didn't exist in the 1990s (it probably did in luxury cars - I never drove them), then moved into mainstream cars, and now the fad is dieing.

They definitely were in (some) cars in the 90s. Probably in the late 80s too. I did IT work for my school district in the 90s and I remember our vans had them. Realtime (digital) MPG displays were showing up around the same time.

My parents had the same model van, but an earlier year; we didn't have the fancy digital compass mirror, just a suction cup compass ball.

they used to sell the suction cup things in my neighborhood in the early/mid-80s. I remember having one to go take a walk in the woods nearby.

I think I should be able to add it to the iDrive. How much did BMW save by removing it from the mirror? $0.05 / mirror?

You'd also need to mount them facing up, pretty inconvenient when driving

Have you not seen these things? It's a weighted magnetic sphere floating inside a clear plastic outer sphere. Anywhere you can stick the suction cup to, the compass reading will be roughly horizontal.

Here's an example https://www.amazon.com/example/dp/B0FZSFWJQR that I'm not endorsing.