I think bugs in the MMU hardware or the kernel accidentally configuring the MMU to allow access across processes that isn't supposed to be are quite rare.
Sure, but I think illegal interprocess memory accesses is a fairly narrow definition for "access[ing] memory its not supposed to". There's plenty of undesirable memory accesses that are possible without needing to cross process boundaries and I don't think the OS does that much to solve those outside of currently niche hardware.
It might be undesirable to you, but you haven't specified this to the computer. Process-boundaries are one way how we specify what is allowed to touch and what not.
OK, sure, but there's no reason you can't extend that argument to in-process improper memory accesses either. free() is you specifying that a particular bit of memory isn't supposed to be touched any more, malloc() is you specifying that some amount of memory is legal to access, etc. Language runtimes, inserted/compile-time checks, etc. would be analogous to the OS/MMU here.
Yes, but this is not across a trust boundary. Since these are in the same process/program. Rust "only" applies checks at compile time, it doesn't enforce security.
Not sure, if I'm clear. Rust is like cooperative multitasking, nice but not guaranteed. My claim here is, that we actually want preemptive multitasking.
Maybe? If we try to backport it to the current hardware/software. It would be an improvement to configure the MMU to enforce boundaries below a process.
However my point was that not every allocation is a trust boundary. What a program does in its own memory doesn't matter at all, this is gone in an instant. Everything that matters is I/O and this goes through syscalls, so there security can be enforced.
Why do you care about corrupting process memory? The memory state itself is totally irrelevant. What annoys you is when it e.g. deletes a file it is not supposed to. Would you rejoice when the file gets still deleted, but the process memory is totally fine? Of course not. The only thing that matters is the deletion of the file, you don't actually care about the memory safety. Thus, what you actually want is the computer to know that the file is not supposed to be deleted, when you have that, the memory can be trashed like the program likes to.