This is a point I was going to make with Prust being the prefect example. Nobody wanted to pay Prust because the Rangers would replace him with cheaper options...and, unfortunately, we got what we paid for in Asham, Halpern, Pyatt, etc...
You may be able to slot Zuc on the 2nd line RW...but where do you make up Callahan's lost goals? This team has needed more goal scorers for years and now we're going to get rid of our 2nd best scorer. And let's talk about grit. Callahan's one of the few forwards who plays a gritty, physical style. The last 2 seasons Callahan led the team in hits. Last season he had 154 and only one other forward topped 100. So now this team that hasn't been able to find goal scorers and has been too soft is going to get rid of it's second best goal-scorer and best hitter...and replacing that production isn't any cause for concern.
BTW - Over the last 3 seasons, the difference in goals scored between Callahan and Nash hasn't been as great as you might think. Nash has scored 30, 21 and 9 (counting yesterday's game). While Callahan has scored 29, 16 and 7.