That's not true, though. If memory safety is the only thing that you need and C doesn't have, there are products that offer memory-safety proofs for C (e.g. https://www.trust-in-soft.com). You do need to add some lifetime annotations and may want to change your code here and there, but it's overall much cheaper than a rewrite in a new language. I believe such solutions are more popular, too; few companies - even and especially those that care primarily about correctness - are crazy enough to justify a rewrite of an existing product just for memory safety.
> Paying that cost and not eking out the maximum safety benefit that's practical... Seems a bit dodgy to me.
The cost is not the same, though (and if you only consider rewrites, as I said, there are better options). It sounds like you're saying that any cost is worth any added safety. I don't think that's true, but if it were, then something like ATS is probably what you're after (it isn't, though, because it's practically nobody's choice). Anything short of that is some compromise between cost and safety. Rust's compromise is just different from Zig's. They're both significantly safer than C and a far way off from ATS. Everyone seems to agree that the sweet spot is somewhere on that spectrum - not C, not ATS - but there's no agreement on where.