I agree completely, but I should point to a problem: it's easy to implement unsound virtual machine on top of sound language. When developers uses indices to simulate references, to circumvent borrow checker, they are doing exactly that. Then again, a sound façade can be built around unsound code, like `indextree` crate does:

> This arena tree structure is using just a single `Vec` and numerical identifiers (indices in the vector) instead of reference counted pointers. This means there is no `RefCell` and mutability is handled in a way much more idiomatic to Rust through unique (`&mut`) access to the arena. The tree can be sent or shared across threads like a `Vec`. This enables general multiprocessing support like parallel tree traversals.