> think about the lengths to which Apple has gone to build a private cloud infrastructure

None? I don't know what you're referring to. Apple's not running any kind of cloud on my private device. Their Private Cloud Compute is about running inference on Apple's cloud but with privacy guarantees -- not on consumer devices.

> It's one thing for them to offer that as a do-gooder service to their customers, which it certainly is.

Again, no idea what you're talking about. Who's doing good to who? Apple's providing a service to get you to buy their hardware. Consumers aren't doing good for other consumers.

> why wouldn't they use spare compute at the edge for inference? It's smart.

You're talking about on people's devices? Because it would use up their battery and heat up their devices and connectivity can be spotty which leads to terrible latency. I mean, that would be as bad as secretly mining Bitcoin on people's devices. How would that ever be smart?

And Apple's in the business of selling hardware. Not helping people save money by sharing hardware. You can't even share an iPad with different logins for family members.