Same reason people walk outside without bodyguards and "bet" on coming back home alive despite the fact that nobody ever gave them such a guarantee. It's not insane to expect the future to bear some kind of resemblance to the past without some contract to guarantee it, is it?
Well you used the example of a car and I think it would be insane to use a component in a car when the manufacturer tells you it's not designed to do that and I can't possibly guarantee that's going to work just because you think you tested it and it seemed fine.
Your example about going outside isn't covered by a technical specification with a long list of guarantees formally agreed between two parties so I don't really see how it's comparable.
I don't believe so - as long as there is a version change and it's documented in the change log, which I aim to do with optimisations I write.
If you're using undocumented apparent behaviour, please check the change log when upgrading, or ask us to document the behaviour. But as already described, we may not be able to guarantee what you want while keeping other things you depend on like performance!
I'm open to ideas on how to create understanding both ways on what the user wants and the compiler is able to provide?