Buying out the final year only saves the Devils money because it's a 35+ contract. I could see them convincing him to walk.
It's not mileage, it's just that Zubrus was never a great player and players who aren't great tend to slow down at Zubrus's age. Like Edge said, he just doesn't get in on the forecheck as fast and so that really limits what good he's doing out there. He's still a smart player with the puck and on defense but he's basically contributing at a 3rd/4th line level at this juncture. I'm not concerned about the salary, but I am concerned that coaching/management usually takes at least a year what shrewd people can see in 20 games - this is why old players suck, it isn't just because they start to get worse, it's that the coaching staff doesn't notice until it's already cost the team points.