Once they re-sign Hamilton they have 10 forwards (including Savard, so 9), 5 D, and a #1 goalie. They could fill their #6D from within, probably with Morrow. Subban could probably step in and back up Rask, or they get some bargain basement guy out of free agency. Campbell and Paille were nice fourth liners for them, but guys like Khoklachev and Spooner are looking pretty close to making the jump, and Bergeron and Marchand are young enough to handle an increase in their PK time. They could re-sign Hamilton for $6.5m, promote a couple of kids to fill out the top9, and then deal Ericsson for the room to troll the end of summer free agency market to fill out the bottom three or four spots.
I think Boston has just reached that point where they're going to slip back a bit and need to re-load with youth, and shed some vets whose deals are getting too cumbersome. Letting Hamilton walk for a handful of picks isn't going to help them beyond the next couple of years, and it's questionable it does that considering the hole he leaves on their blueline.