Boston has 5D making more than 3M, and two around 1M. Most successful teams have a strong top 4 or 5 making real money, and after that it's cheap, ELC-level contracts. I'm not sold on Harris belonging in the upper grouping, so giving him a raise that puts him on course for a top-4 role when he hasn't shown this yet seems premature.
He might well find a top-4 role, but if he doesn't, he'll be overpaid and Habs will likely have to move him or not qualify him. He has neither the Edmundson/Chiarot/Savard type size and shutdown ability, nor does he seem to have high-level offense, so a top-4 role isn't a given.