I'm honestly not convinced that immutability for local variables is a particularly useful feature. Within the scope of a single function, it's usually pretty easy to see if a variable is being reassigned or not; there aren't the same kind of programming-in-the-large preventing-spooky-action-at-a-distance benefits that come from immutability across API boundaries. You increase the language's complexity and learning curve, for a benefit that's rather speculative and unclear.

(The exception is closures; prohibiting a variable from being reassigned after it's been closed over is useful because anyone reading the code may expect one of two conflicting behaviors depending on context, and so it's good to instead write the code in a way that doesn't have that ambiguity. Java got this one right.)

A post articulating this point, by the person primarily responsible for Rust's shared-xor-mutable architecture (so presumably he has some idea what he's talking about, though note that his argument did not carry the day): https://smallcultfollowing.com/babysteps/blog/2014/05/13/foc...