The entire premise that you need an MMU is about invalid memory accesses. Safe rust does much the same job with much finer grain and at compile time not runtime where dealing with page faults can lead to some strange loops.

No need for IPC or syscall marshalling when you can simply do function calls without the hassles of MMU permissioning.

Like why is a microkernel needed today for this sort of thing, if its purely for security of memory then... well there's other ways of solving that problem completely outside of hardware memory protection schemes at this point in my mind.

We're so steadfastly stuck in this mindset of needing memory protection and pointer protection (CHERI) because of the insanity that C and its like bring to the table. Restricting the software to safe rust does much the same job. Memory is protected, lifetimes are protected, and better still data races are protected.

E.g. see rtic-rs and how tasks are written. Without unsafe, no memory is accessible outside what is given explicitly to the task. And checking for unsafe rust can be done in an automated fashion at compile time.

Or maybe I'm totally wrong which is quite possible!