There are definitely legitimate situations where that is the case, but I do not think this is one of them. To quote the reason for the rejection (from here):

I accept that you don’t want to be involved with Rust in the kernel, which is why we offered to maintain the Rust abstraction layer for the DMA coherent allocator as a separate component (which it would be anyways) ourselves.

Which doesn’t help me a bit. Every additional bit that the another language creeps in drastically reduces the maintainability of the kernel as an integrated project. The only reason Linux managed to survive so long is by not having internal boundaries, and adding another language complely breaks this. You might not like my answer, but I will do everything I can do to stop this. This is NOT because I hate Rust. While not my favourite language it’s definitively one of the best new ones and I encourage people to use it for new projects where it fits. I do not want it anywhere near a huge C code base that I need to maintain.

These do not sound like the words of someone who had been on the fence but was finally pushed over to one side by the last patchset in a sequence.