I'm as anti LLM use as they come, but this appears to be migrating libraries from already funcitoning C++ code. In the case of your hypothetical I suspect the course of action will be "shelve this library port until someone with domain expertise and Rust experience can look at it". Its not like he chucked the whole codebase at the GenaI gods and said "Port it to Rust!".

> what happens if they hit some complex production issue

they learn Rust

it takes a couple of years

it's not that hard.