The argument Rust devs make is that most of the time you would not need to use unsafe code and when you do, being explicit about it would make you more careful and think twice about it.

To me it makes sense. And the example you give here is very relevant. First you'd try to do it within the standard language bounds and only when you realize you can't do it that way, I'll resort to unsafe code. But now your very aware that this part of the code needs to be treated why extra care. So, to me, you're not completely back to square one.

Nicholas Matsakis make this very point near the end of this talk: https://www.youtube.com/watch?v=9wOzjbgRoNU

I would even add, if care is taken to make that unsafe code really small it can even been generated by Coq for instance as stated in some comments here.

That said Rust might not be the best out there for the job but IMHO it shouldn't be dismissed to fast either. It is similar enough to C++ to allow a less painful transition for devs with the domain knowledge.