He's so replaceable that the rumored return is Chris Stewart, who is a downgrade in every way imaginable, most notably in intangibles and attitude. Please name some of these players who can replace what Callahan brings and be attained without trading away huge pieces.
I don't know why you're bringing up Boston, they have plenty of player like Callahan. The Rangers have 1. Nobody is saying have 23, but the Bruins have far more than 1.
Callahan's reported demands are insane and untenable. That's what this all comes down to. Eveyrthing else is mental gymnastics to try to make this seem like a good deal when its really just the best of a bad situation. Callahan is not easily replaceable. There aren't many players who bring what he brings, let alone ones that can be had. The team needs him and more players like him. Not a whole roster, but more than 1 for sure. The contract demands are just insane. That's all it comes down to.