But Rust isn't available everywhere, so surely someone will be losing out.
If you're running software on a Pentium 2 or a DEC machine then maybe you're out of luck, but there aren't that many platforms out there that are still in use that will run into problems here.
Perhaps if you're stuck on a long unsupported version of Windows or CentOS and need the latest version of Python packages you're in trouble, but you already were when you got stuck on a legacy platform anyway; I doubt you'd be updating your dependencies to a version where this is a problem if your OS is that outdated. You'll have the same problem with your dependencies needing a C compiler more recent than GCC 4.
This is no big deal for us, but it can be quite confusing and frustrating for users who haven't written compiler code before.
What Python dependency managers need to improve, in my opinion, is for their toolsets to inform users about native dependencies when compilation is required.