or the kernel releases with broken Rust
This is what I don't believe. I think what will actually happen (or could at least), is:
C dev that refuses the learn Rust: "Hi, here's a change to the DMA API."
Linus: "Can you fix the Rust code before I merge this?"
C dev: "Ok, Rust devs it's your job - can you fix it?"
Rust devs: ""
C dev: "Hello? Where are you?"
...
C dev: "Can we just merge it now?"
Linus: "No we need to fix the Rust."
Again, to be 100% clear, I think that this shouldn't block Rust. We should just expect the C devs to learn a bit of Rust (seriously if they're writing Linux DMA systems they are easily bright enough to do it). But pretending that they won't have to to keep them happy seems disingenuous.