Linux maintainers are unwilling to get rust into the kernel, so some rust folks decided to start writing a new kernel with same ABI. This allows them to make new architectural decisions. An example being their “frame kernel” (something between a monolithic kernel and a microkernel).
If I may say, it’s more legible and the tooling is way better, right off the bat.
The Linux kernel already allows proprietary modules via DKMS, and a handful of vendors have been using this for decades, so this is no different. Case in point: NVIDIA driver, and Android