> If money remains backed by AI goods and services

Why do you think it would? If 99% of people never use those services, why do you expect they'd continue to exist? Just to justify the existence of some temporary class of worker to provide services that, for some reason, aren't fulfilled by the AI?

> Your diamond example is just swapping which goods are scarce.

Not at all. If you're starving and someone offers you the last diamond in the world for a weeks worth of food, its unlikely to be a worthwhile trade since without the food you'd die, and then (to you), the diamond is entirely worthless. Scarcity raising prices is a fundamental law of our current economic system, its not a fundamental law of reality