I don't think he has the offensive upside to make more than 3.5M, he had 22 points last season and was -7, usually to get more you need to do more than just be a defensive player(I have no clue how Komisarek got $6M or whatever Burke threw at him).
Players with comparable point output:
Bieksa - 22 points, +32, 4.5M cap hit
Foster - 22 points, -12, 1.8M cap hit
Greene - 23 points, -23, 3M cap hit
Rosival - 21 points, +6, 5M cap hit
Big contract differences but the main difference between these guys and Schenn is that they have been in the league longer. I would go as far to say that they each have qualities that are better than him, besides maybe speed when it comes to Greene and Rosival. Maybe in a few years he will be good enough to get something higher.