I'm not sure passing everything by reference probably would gain much in a systems programming language in the vein of Rust: one would still need some syntactic method to distinguish between shared ("immutable") and non-shared ("mutable") references. Also, Rust-style static reasoning cares about things actually being pointers, to allow returning references safely. Lastly, being able to move things around is important too, it is a major part in Rust creating zero-overhead abstractions.

I'm not sure exactly how MLKit works, but I glanced over https://github.com/melsman/mlkit/blob/master/doc/mlkit.pdf and it seems to me that its regions are somewhat less static and that it doesn't attempt to handle concurrency. I could be wrong on both counts!