Re: Why not just develop a "LinuxR"?
One word: Cost. Look up "cost of redevelopment of the Linux Kernel".
There is also time ... how many man-hours over how many years do you have to devote to this project?
And then, when you are done, you'll have a Linux Kernel in Rust that is several-to-many years out of date, depending on which kernel version you are targeting.
See ReactOS for a project similar in scope.
Note that I'm not saying it couldn't, nor shouldn't, be done! That's up to you. I'm just pointing out (some of) the logistics that might get in the way.