First, I think Bos has to retain on this. Second, I'm not comfortable with only those 3 lhd in the playoffs, maybe only griz and Moore next year.
I think a smaller trade on Carter only, maybe....

That really isn't very cheap.

Carter is on pace for 41 points this season which is 6 points less than Backes was on pace for in 82 games last year. Sure Backes has had a poor season, but production wise it was mostly the start of the year when he scored 3 points in his first 20 games. Since then he's had 10 points in 20 games. Carter's had 12 in his last 23 games, so it's more or less the same production. Sure Carter's a bit better, but his contract is even longer and they're both 34 years old.

Krug's the leading scorer in terms of points in this trade and he's on the side with the 4 youngest players with the most potential if you count the picks.

Toffoli is the only thing added on from LA and he's barely better than Heinen. I don't think you'd find it cheap to trade Krug and 4 potential top 9 players for Heinen, I'd think you'd call it an out and out ripoff.

I don't see why people have interest in Carter, he's on the back end of his career. The Bruins don't need to add another 5m+ skater who's 34 years old, they need to get rid of one (Backes) without taking on any salary so they can pay McAvoy, Carlo and others. At worst the team sells Backes @50% without taking any cap on by trading a 3rd round pick or prospect that didn't work out for the team, yet has value to another franchise kind of like Vatrano ended up.

To counterpoint.

Carter - you are not taking into account him being on a better team and possibly on RW on the Bergeron, Marchand line. He is still can be a prolific goal scorer.
Krug - Leading scorer who is still a -5, that can't be ignored, if you score a 100 goals and are -100 you aren't really contributing quality defensive play. He has value because of his points, that's why we should trade him now.
Toffoli - Way better than Heinen, Heinen will never be any better than a 3rd liner, I've been saying this for over a year now. Toffoli would look great on our 2nd line.
Carter's Contract - Valid but I see getting rid of Backes contract a better option because Carter can be amazing if on a good team.

If Carter plays RW on the first line, the problem then becomes that Toffoli will be bumped to the third line, in which case I'd rather have Heinen because he'll most likely be cheaper next year. If not, then the team is paying 5 million for a third line center, which is probably too much and doesn't solve the Backes problem. I think either Carter or Toffoli could be a good addition to the team, but not both.