They find it immensely useful in practice only when the safe subset is useful. In C, you could say that the same definition of memory-safety exists, only the safe subset is empty, and in that case people don't find the fact that C could be described as memory-safe in that way useful at all.

And that's exactly my point. The safe subset of Rust doesn't sufficiently cover the very things that I choose a low-level language for in the first place, and the parts it does cover I can do at least as well in even safer high-level languages.

Now, I'm not saying there aren't programs where Rust's precise mix of safety and unsafety is better than the alternatives. For example, I've never written a browser rendering engine, and it's possible that I would find Rust to be the best fit for that task. I don't know that I would, but I also have no experience with that domain to suggest that I wouldn't.