Welcome to the problem the Scientific Python community has been struggling with for years. Providing binaries for a plurality of platforms brings challenges with it, but I reckon on the major platforms Pydantic will be fine as they won't have hard to compile parts.
But Rust isn't available everywhere, so surely someone will be losing out.
Rust is available for almost all common platforms. There are crates that don't run on some platforms (i.e. relying on CPU intrinsics that aren't available cross platform) but there's a Linux, macOS, Windows, FreeBSD, NetBSD, and more. It's supported on x64, ARM, MIPS, PowerPC, RISC-V, and s390x. There's even a rustup version for running the compiler on i686 Android.
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platform anyway; I doubt you'd be updating your dependencies to a version where this is a problem if your OS is that outdated.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.
But with python the custom is to ship prebuilt binaries. And there won't be prebuilt binaries for plenty of normal-ish platforms, like Alpine Linux. So those users will now have to set up local complication for the first time.
This is no big deal for us, but it can be quite confusing and frustrating for users who haven't written compiler code before.
It looks like pydantic-core is distributing musllinux wheels, which should work fine on alpine. Fwiw tooling like cibuildwheel makes building and publishing wheels for all the common platforms fairly straightforward now.
My experience is that though the error messages Python produces are quite obscure and verbose, the installation of the necessary packages went quite smoothly. I've pkg added Rust in Termux on my phone, and it that works I don't think getting stuff built will be all that challenging.
What Python dependency managers need to improve, in my opinion, is for their toolsets to inform users about native dependencies when compilation is required.
But Rust isn't available everywhere, so surely someone will be losing out.