Incidentally, until C11 "volatile" didn't guarantee a proper memory barrier execution. It does now, so your analogy has a hidden truth to it.
I'll just counter-argue though, following your analogy, that the same way we see the C standard as an exposition of the language strengths and limitations, so we should look at the ISA. I look at your use of the word 'contract' as being a bit naive, or perhaps too convenient, since we are discussing the limitations of modern hardware.
But I won't argue we would be living in a crazy world indeed if our processors could run unchecked unpredictable code. For every optimization that takes away execution correctness, a safeguard has been implemented somewhere. But I don't think anyone argued against that. It was instead your denial of the existence of this problem that spurred the discussion.