Borrowing isn't for mutability, but for memory management and limiting data access to a static scope. It just happens that there's an easy syntax for borrowing as shared or exclusive at the same time.
Owned objects are exclusively owned by default, but wrapping them in Rc/Arc makes them shared too.
Shared mutable state is the root of all evil. FP languages solve it by banning mutation, but Rust can flip between banning mutation or banning sharing. Mutable objects that aren't shared can't cause unexpected side effects (at least not any more than Rust's shared references).
Owned objects are exclusively owned by default, but wrapping them in Rc/Arc makes them shared too.
Shared mutable state is the root of all evil. FP languages solve it by banning mutation, but Rust can flip between banning mutation or banning sharing. Mutable objects that aren't shared can't cause unexpected side effects (at least not any more than Rust's shared references).