They're not "forced to use Rust". They are maybe forced to work with Rust developers of whichever subsystem needs to be updated, but that would always have been the case with the C developers of whichever subsystem needs to be updated too.
The same situation of course also arises between C-only subsystems, but then the natural solution is that you have to go and understand system B well enough yourself that you can make the necessary changes to it and submit them as part of your patch. In that situation you are "forced to use C", but that's a free square because you are always forced to use C to contribute to Linux code.
So if the maintainer of subsystem X can be forced to work with the rust developers of their own subsystem, then that rust developer just got promoted to co-maintainer with veto power. Effectively that's what they'd be, right? I can see why maintainers might not like that. Especially if they don't think the rust dev is enough of a subject matter expert on the subsystem.
That isn't "effective co-maintainership".
It was nothing like making changes that cut across into another developer's C++ code (hell, I would even update their python interfaces/consumers too). That was temporary coordination. The python part was much more frequent and required much more detailed understanding of the internal APIs, not just the surface.
Having someone else responsible for the python part would have come at a huge cost to velocity as the vast majority of my changes would be blocked on their portion. It's ridiculous to imply it's equivalent to coordinating changes with another subsystem.