Usually rewriting something in Rust requires nontrivial choices on the part of the translator that I’m not sure are currently within the reach of LLMs.

Sure, but the LLMs will just chain 14 functions instead of 7. If all C code is rewritten in Rust tomorrow that still leaves all the other bug classes. Eliminating a bug class might have made human attacks harder, but now with LLMs the "hardness" factor is purely how much token money you have.