He definitely messed up by letting McElhinny go, but the fact of the matter is we'd be trading assets to replace him either this off season or in the near future, with him being 37 years old. Campbell is significantly younger, and arguably better.
Matt Martin was making 2.5 Million/Season, we can't afford that on our 4th line (we can barely afford to pay 3rd liners that much), especially considering that he can't kill penalties. Clifford is two years younger, was brought in making one million less than Martin, and has been able to slot in on the third line when needed, and has been known to kill penalties.