Unix philosophy has long been that the person with physical access to the machine owns it. This would only be acceptable for a very specific class of compute jobs where nobody cares if you hack them. Like computation for charitable purposes. You could never host peoples personal or business data in this. Furthermore bandwidth would be limited and unreliable, as would power.
Physical access means you can directly measure things that would otherwise be secret. Nothing stops you from reading the contents of memory wholesale, for one simple example.
If you open the case, the system deletes the keys and you can say bye-bye to your contract.
tl;dr - modern physical security is much more than microswitches.
This seems like a very challenging environment for tamper detection.
See, that's the problem every time this comes up (and it's come up about as often as micropayments). By the time you get done accounting for the unreliability, hostile end-users, lack of access, customer support, low-grade obsolete inefficient un-fixable infrastructure, increased avenues for attack, duplicate processing, super-high latency, super-low bandwidth etc, you've created something that's several times slower, more expensive, more unreliable, more complex, and harder to upgrade than if you'd just stuck it all in a datacenter where you don't have to propose insane mechanisms like double your computations.