I'm not sure why people have wanted to move Rust these past couple years.
He's a good player. He just had a career year, I'm sure he'll fall back to earth and become a 15-20 goal scorer and 30-40 point player again, but at $3.5M given all the intangibles he brings, that's not bad at all. He's a threat on the PK, he's solid defensively, he can play anywhere in the lineup which is severely underrated given the catalogue of guys that fit with Crosby, but not Malkin or vice-versa, and he's productive.
Of course he isn't untouchable, but he wouldn't be the first guy I'd move unless I was blown away. The only untouchable player is Crosby, and even then, if Ken Holland calls me up and offers McDavid for Crosby, I'm taking that deal. There's like a 1% chance Crosby is ever traded, he'll be a Penguin for the rest of his career most likely. Malkin and Letang, are probably 75% likely to play here their entire careers. I wouldn't move them at this point in their careers, they're still good players and the return for them would be severely underwhelming. Guentzel, Zucker, Rust, Dumoulin and Marino are all guys I wouldn't move unless I knew I was winning that trade. Everyone else is expendable.