Rust doesn't add anything that increases maintainability beyond memory safety. This is nice, but it's not even 1% of what maintenance work looks like. Maintaining a decades old code base means adding features without blowing up the beast. This involves: (1) digging through the code to find the area that does something close to the new feature, (2) trying to understand what is going on, (3) figuring out the style of it for conformity, and (4) slapping that bad boy in.

Of these, (4) is by far the easiest part and the only one Rust will help you in. The digging for understanding is far more time consuming. The other parts require art, discipline, and linters.