Chara has clear weaknesses at this point, and I think folks in this thread have pointed them out well. That said, his size and ability to clear space around the net is something the Bruins simply do not have an internal replacement for at the moment, and it's a role Chara can continue to play. If he's willing to take 3rd pair money and play reduced minutes, having Chara around is a net positive. I actually think Krug's production is a much simpler problem to solve, as a Krecji-McAvoy pairing on the powerplay should be nearly as effective, and while we'll lose Krug's offensive capabilities on 5v5 those are offset by a likely improvement on the defensive end. With the forwards being pretty well set if DeBrusk is retained (although I'd have no problem firing Ritchie into the sun and replacing him with Frederic), I'm with PSK in that I think the best course of action is going for a better 2nd pairing D-man. If you get someone with a bit more heft, you can avoid scenarios we saw (repeatedly) in these playoffs where there are two defenders out on the ice who are simply incapable of keeping players out of the crease.
That said, if we're firing people into the sun, I'd strap Connor Clifton to Ritchie. Vaak, Zboril, or someone needs to step up and take his job, because he really shouldn't be anything more than a 7th or 8th defenseman.
I really disagree about Clifton. As others have mentioned, this team is excellent defensively, but they need to get bigger on D (thank you Krug, I hope you get paid big dollars elsewhere) to become more matchup-proof. But they also need to find some more offense. Cliffy isn't big, but he isn't as small as Krug either, and he definitely brings offensive skills. Moreover, he's young, on an ELC, and signed thru '23.
Looking at the LD UFA list, Gustafsson, Edmunson, and Hutton look interesting to me. Edmunson is your Chara size facsimile at 6'4'', 215 but probably brings the least offensively and will probably get a AAV salary increase on his $3.1M, Gustafsson has the ceiling with his insane 18-19 season certainly enticing but he's also the smallest of the 3 (6'0'' 197), and Hutton seems like a really nice player that can probably be had for a reasonable price and is big but not gargantuan at 6'2'' 206. I don't know these guys well enough to say whether they could slot in as the LD in the top pair with Mac, or give the 2nd pair some real quality.
I love Krejci but he hasn't provided good enough ROI during the term of this contract. He's only cracked 60 points twice in 5 years and he's definitely towards the bottom end of point production for players earning $7M-$8M. He's only signed thru next season, but I think that will be an opportunity to start building a new salary structure, with Bergy only signed for another season after Krejci allowing a pretty serious reconfig if Donny wants to.
If the Broons want to get more goals from the forwards, I'd rather see them build from the top and see if the kids can provide some energy and quality in the bottom 6. It's not ideal, but the Panthers likely won't resign both Dadonov and Hoffman. A high AAV, shorter years contract could definitely work really well for 2 years before decline kicks in. I haven't followed him much recently, but what kind of salary will Taylor Hall command this offseason? He made $6M this year, so probably too rich for us, but he would certainly move the needle.
Then there's Rask. That changes things immensely, as his leaving would free up salary but would immediately make our D look worse.