You can also trivially model a thermostat using lego bricks. However, you don't need a lego-based model to understand a thermostat

That's a complete straw man. I never claimed that you did. What I said was: "a utility function models its behaviour pretty well" - which is perfectly true.

I'd go so far as to assert that you could (but shouldn't) model anything that is computable in a way involving a utility function.

Any computable agent. If it iisn't clear how to decompose a system into sensors and actuators, representation in terms of a utility function is not so useful - because it is not unique. It is convenient to use utility functions when you want to compare the values of different agents. If that's what you are doing, utility functions seem like a suitable tool.