"Trivially known to be true" until the code evolves making your unstated assumptions not hold and everything breaks, often in complex and unintuitive ways involving interactions across modules. This is why these automated soundness checks are valuable.
"until the code evolves [...]"
That is already a desirable place to be, where you managed to get a working implementation ready to evolve. My issue with opinionated languages like Rust is that they make development more expensive. I then afford to pay the necessary work-effort for fewer projects than I otherwise could if I was to focus more on the problem(s) at hand instead of that and other mandatory constraints forced upon me by the compiler. I very much want my development tools to limit themselves on being tools, to assist me on the part of the problem I chose to focus on with little to no cost paid for their usage. I want to be able to focus on prototyping some working solution first, and only then, if the project's needs really warrant it, to switch on paying the development cost for other aspects, be it safety or whatnot.