The problem with always saying X player isn't worth the money imo is that, who are we waiting to sign? You don't get bonus points for having cap space available. I can see the duration being a problem down the line maybe, but you can always buy out the contracts. It seems to work for other teams.
Callahan's contract will not provide value in Year 1. It will not provide value in Year 6. Ryan Callahan belongs to the class of player who is always overpaid upon reaching UFA because he never takes a shift off. His best years are probably behind him already, and he's likely to become more injury-prone as he ages.
The Devils can't afford a contract like this because they have too many other problems on the team and hustle isn't one of them. Callahan's contract will be Clarksonesque - if he makes it to July 1, his contract will immediately be one of the 5 worst in hockey.