Sweeney will get Krug extended. Chara is nearing the end, might be one more season, after that who knows. Bergeron will be 34 nearing 35 when next years playoffs start, Krejci will be 34 just in mid-April.
They'll need top guys and leaders in that 27-31 age range. That's Marchand, that's Krug. He's part of this core and unless Sweeney is blown away with an offer (think an age-comparable top line winger or center), Krug will be part of the mix for quite some time year. He'll wear a letter for the Bruins before it's all said and done.