I don't think pattern matching and union-types makes narrow typing useless. Rust has both pattern matching and union types but still implements some specific forms of control-flow based typing.

For example, consider this Rust snippet :

    pub fn positive1(x: isize) -> Option<usize> {
        if x > 0 { Some(x as usize) } else { None }
    }

Unless I'm mistaken, without narrow typing, this cast would be impossible, and there would be no way to avoid the redundant runtime check caused by try_into().unwrap(), that is not even optimized unless you trick the compiler.