It seems a microkernel architecture allows for the kernel to be as small as possible, with the scheduler and drivers and everything written in a safe language in user-space. In that case, it is anything other than vanity that says the kernel is better off being written in a "safe" language with all the safeties turned off, compared to a unsafe language?
2. You don't have to turn off safety checks in all of the code. Only having to audit the unsafe code is still a huge win. You're vastly overstating the amount of code that can't be safe.
3. Fuchsia has many components outside of the kernel that should have been written in safe languages but weren't.
4. It's funny that you use the word vanity. I think it perfectly describes the attitude that you're smart enough to write a nontrivial project in C/C++ without any of the many classes of preventable bugs that safe languages fix. Or that a few parts of your code requiring unsafe behavior somehow elevate the whole project into an elite class that doesn't need safety.