What extra effort? If you match upstream it's a drop in replacement and anyone who cares will use it. If they don't it's because they don't care.
This is primarily around Rust filesystem drivers. If you want to run a filesystem but it's implemented in Rust, you'd have to use that fork. If another person had the same issue (say a GPU driver that some maintainer decided they didn't like because of the country of origin or some other petty reason), then that would have to be a fork as well. Suddenly, you can't use that GPU with a Rust-implemented filesystem, because you have to pick one of the forks, or you have to make your own merged kernel from the two.
"Just fork it" doesn't work here. It's a logistical nightmare.
Talk about aggressively missing the point.
Rust developers are promising that rust in the kernel should have no impact on c in the kernel.
If rust in the kernel has no impact in c then you should be able to remove the rust from the kernel and move it to it's own project, since it will have no impact on the c either way.