Having binary blobs linked into your kernel is a maintainability nightmare. You’re allowing a third party to link their buggy drivers into the heart of your platform. It breaks any security model you have, and brings with a bunch of bugs that are impossible to debug.

Nvidia were the worst offender and it culminated in this:

https://arstechnica.com/information-technology/2012/06/linus-torvalds-says-f-k-you-to-nvidia/

The Rust kernel devs are …

1. …asking the maintainers to lock down APIs which the C devs purposefully leave malleable, in part, to avoid binary blob drivers being feasible.
2. …asking maintainers to accept code into their subsystem whilst being told, you don’t need to know Rust to an expert level…trust us. Cross language interfaces always have nuance and make good attack vectors. Understandable that maintainers are cautious.
3. …creating quite a lot of hassle for no a lot of improvement. Systems are only as resilient as their weakest components. The cross language interface is always going to be weak. Introducing a weakness to get improvements probably only succeeds at making the whole weaker.